I'm new to electric guitars, but I understand that all the different models out there have various signal-modifying electronics shoved somewhere inside the body that modify the pickups' signal before getting to the output jack.
Why would anyone want that? Wouldn't it be better to just output a "raw" signal and then have my choice of what signal modifiers I hook up AFTER it comes out of the guitar's jack?
Its just another point of tonal control basically. Raw signals can be very intense, and this helps to smooth the dynamics out a little bit. As well as providing a rough EQ with your tonal knobs.
Generally those knobs are associated with one of your pickups, usually with one that controls the full output as well, and as such will control their output by raising or lowering the gain on that pickup, or all of them, respectively.
Some knobs are balance knobs which means that turning them one way changes the bias between the two signals it handles, such that turning the knob clockwise will boost your neck pickup and turning counterclockwise will boost your bridge pickup. For the most part, these knobs have a threshold of about 10-20% minimum signal from each pickup, meaning that the most biased a signal can be is 90-80% one way or the other. This solves being too bassy or too treble.
There have been several musicians in the past who have only had a volume knob or no knobs at all on their guitars. They aren't necessary, but they don't hurt either, its all in how you choose to model your sound.
see http://www.diyguitarmods.com/guitar-wiring.php for more information about guitar electronics.
It's more about practicality than of necessity.
Imagine a guitar with one pickup and no volume or tone knobs. You're right, this would produce slightly less noise (probably less than the noise floor you'd get on any recording - but in theory any component passive component will add some noise). However, you would probably want control your volume/gain sooner or later - thus you would get a volume pedal.
If you wanted to add a pickup (to get a more mellow, smooth sound for soloing or something like that - or maybe you want to lay some jazzy chords during a verse) you would have to add another output to your guitar. Then, either patch both cables to a AB-Y merger/select, then again to the volume.
What I'm getting at is this: Because most guitarist have found that they want to control their tone/balance and their volume/gain, most guitars come with that as default. The same applies to pickups, because most guitars want to be able to get the sound from both the bridge and the neck position (and switch between them, but only connect to one out) a pickup switch is standard.
There are plenty of exceptions.
Have a look at Eddie Van Halen's signature (one picup, volume only):
Another consideration: assuming your guitar has at least two sets of pickups, if you didn't have what's essentially a mixer on/in your axe, you'd need to pass more wires to the remote preamp. That not only increases the complexity of the patch cord but increases the risk of crosstalk and interference pickup.
It's a matter of convenience, and of tradition.
On the earliest amps, all the performer had for output control were volume and tone. The only difference between these controls and the ones on a Telecaster was that the amplifier had a gain stage prior to these controls to boost the signal before any cutting of volume or high end was done. It wasn't until later that the RLC band-pass circuit was discovered, as well as various techniques for employing it that economized on power (for instance, using the filters to induce phase cancellation rather than shunting signal to ground).
Guitarists, who at that time were basically playing modified acoustic guitars with magnetic coil pickups placed strategically in the top (leading to what would become the semi-hollow body style such as on Gibson's iconic L and ES models), complained that this setup was inconvenient; if they needed to make a change to their tone, or simply to silence their instrument (feedback was a serious issue before the development of the solidbody guitar in the late 30s), they had to go over to their amp, kneel down, make the tweak, then play the next song, checking to see if the change was what they were after. In response, custom builders began wiring simplified versions of the exact same controls used in the amplifiers either into metal boxes that could be wired up into the instrument cable, or directly onto the tops of the guitars.
The diagrams for the famous Rickenbacker "frying pan", arguably the first true production electric guitar, at least took the need for volume control into account; the design submitted to the USPTO in 1934 included an inline volume control. Custom archtop acoustics with pickups mounted had master volume and tone controls mounted since the mid-30s. The iconic designs we have today, first the Gibson L-series, then the Les Paul, the ES, the Broadcaster/Telecaster, Stratocaster, SG, etc. all had these controls because they were demanded by players, having gotten used to them on their modified acoustics.
Nowadays, the modern examples of these guitars have the electronics configurations that they do because the guitar simply wouldn't be a Tele, or an LP, or a Strat, without the same control configuration seen on the instruments used by the idols of the guitar world in the 50s through the 70s. The exact reason you'd want to remove them (or replace them with updated, more transparent "active" preamplifier controls), to get the raw sound from the pickups into the amplifier, is the exact reason they can't be removed; it would change the tone of the instrument, and it would no longer sound like the guitar's ancestors.
That said, there are always those that bucked the trend. Nikki Sixx modified his bass to remove the electronics completely, save for a single mute switch. Eddie Van Halen built his iconic Frankenstrat from scratch, wiring it without tone controls in part due to his limited knowledge of guitar wiring. While active circuitry in guitars has seen relatively limited adoption, the same cannot be said of basses, where active preamps are common though still not ubiquitous (still plenty of demand among players for the traditional passive designs).
