This question has certainly been answered, but I want to contribute some of my own thoughts.
Ableton has some of the best instruments in any DAW. That is to say, their built in synths and sampler are awesome. They also provide great effects and mastering tools (I'm referring to the Suite version here, but the same also applies to a lesser degree with all versions).
Truthfully, you would most likely never need anything you couldn't find in Ableton.
That said, plugins are awesome. Here is my thinking on downloading and looking for external plugins.
- There is something creatively uplifting about approaching your production with new tools. They may not be better, but at the very least, there will be new knobs and dials to tweak, and you might come up with new sounds that never occurred to you before.
- This is also great for learning. You could learn everything you needed to know about digital audio production using just Ableton's built in plugins - however, I've found that it is often the case that I find myself understanding something that has been under my nose the whole time in a completely new way when I look at it from a different angle. There are many different ways to present digital audio information, and there are many different audio properties that are exposed in the settings depending on which plugin you are using. Looking at many different plugins also helps you learn for yourself to distinguish between inferior sound produced by inferior plugins, and this knowledge helps you hear your own music better and find sounds that lack quality.
- I like downloading things. It makes me happy.
You can find MANY,MANY plugins at KVRAudio.com, both free and commercial.
I'm going to make a general statement here, and I don't know if its true - it seems to me that in the digital audio software field there are more QUALITY programs (VST's etc) available for free than in any other industry.
It is much easier to find great free effects than it is to find great free instruments (with some notable exceptions), but you get to explore. Also, KVR Audio has a lot of forums where people ask useful questions. Their are more sites like it out there as well, but that is certainly the largest.
There is my thinking on some of the important value of plugins for someone - particularly a beginner - who is working with a digital audio workstation.
Mostly - you can just have fun with them, but I would not spend any significant amount of money until you really know what you are doing.