I'm doing recordings for YouTube videos and would like to know whether a pop filter is really necessary. I, of course, care about audio quality, but I'm on a tight budget. Do I really need one? I'll most likely just be doing commentaries, not singing.
You need a pop filter if the mic is in a direct line in front of your mouth - whether you buy one, or stretch a piece of material over a frame in front, put a sock over it, whatever.
You don't need one if the mic is off to one side, so you can never breathe or 'pop' directly into it.
This would require lifting the high frequencies to compensate; so would depend on the room noise & acoustics.
It's worth noting that many mics actually have a pop filter built-in, in particular live vocal mics such as the SM58. Such filters generally don't work as good as a proper external lollipop-screen, meaning they take away more treble. (The main difference is, foam absorbs more sound than light fabric.) But for speech, this isn't actually such a big deal: most important is a distict midrange. Sure, nothing will sound as clear, full & transparent as a large-diaphragm plus pop screen, but if intelligibility is more important than sound-sweetness then such a dynamic mic might be a better budget-solution than a condenser you have to place further away, or some questionable pop-removal hack.
(However for music recording, I would always prefer condensers, no matter what it takes: in fact I still use my Behringer B1 with a sock over a frame made of bass-strings sometimes, when I've run out of mics for backing voices. It sounds reasonably good, although for a YouTube video the look would be hardly acceptable...)
They're not strictly necessary, but they're certainly preferable. But that doesn't mean you need to buy an actual pop filter! You can easily slip a sock or a cotton shirt over your mic and it'll have close-ish to the same effect.
Pop filters are necessary for anything voice related. It helps with sounds such as 'P' and 'B'; This is called plosives. Plosives is extra air going onto the microphone causeing a very annoying heavy breath/bass sound. If you care about audio quality and you want to be great at what you do, get a pop filter. You will not be disappointed.
For you they are not really necessary since you are just talking.
FWIW they are a nice way to keep dust, spit and other things off the mic element, which helps extend its life time. That being said mics like a SM58 have a built in one so another is not really needed. If you are using a mic with an element you can see I would put one on for protection just to be safe.
They are not that big of an expense and can usually be had for a few bucks if you are on a cheap budget. They may save you from buying a new mic down the line as well.