Is a compressor necessary? ("Another guy told me he never uses compression.")
Short answer: No, it's not for everyone. A compressor is a very useful tool, and just as with any other tool, the better you understand how to use it the more useful it can be. There are a lot of circumstances in which a compressor is the right tool for the job. But that doesn't mean you have to use one. Many people prefer to regulate their signal peaks as much as they can strictly through playing technique. And many amplifiers (especially tube amps) compress your instrument signal a little or a lot just by the nature of the amp design. Read here for more about that. Overdrive and other types of distortion also compress your signal considerably. So under many circumstances it may not be "necessary" to use an external compressor. It's up to you to analyze your particular situation and determine if a compressor, as a tool, can be useful to you. Remember too that misusing a compressor will get you bad results, exactly the same as misusing any other sort of tool. If you apply a chainsaw when you needed a fine dovetail saw, it'll be bad news for your woodworking project. But don't blame the chainsaw!
So why would somebody want to use a compressor? What is it good for, and when? Here are the basic benefits, which vary from one comp to another:
-Peak limiting, to avoid blowing your speakers or hearing unwanted distortion.
-Evening out levels between strings or between soft and strong playing.
-Changing the attack and other envelope qualities of your notes, making them squishier, rubberier, crisper, or other amplitude effects which may be difficult to achieve just with your fingers.
-Emulating some of the tone and envelope qualities associated with older recordings (Motown, early rock and funk, etc.) in which tube and transformer-based recording equipment and reel-to-reel tape recorders got "saturated" by the bass signal. That saturation compressed the recorded tracks.
-Increasing sustain or punch by increasing the average level of the signal without increasing the peaks. This can provide greater perceived loudness, fatter tone, more audible harmonics, and better placement in the mix.
Also, even though some people say they don't like compression because they play with a wide range of dynamics, the fact is that carefully applied compression will actually make the entire dynamic range easier to hear, rather than making your signal sound "less dynamic". This is partially about the quality of the compressor, partly about the type of compressor it is, and partly about how the compressor is being used. Failing to adjust the level of the signal going into the compressor can make anyone hate compression. And some compressors just suck.
Some amount of dynamic control can be achieved just using your fingers, and that is a really important goal to work towards while practicing. However some signal spikes can't be prevented with your hands, such as a resonant peak from an envelope filter, or a flanger or delay set to high resonance. And some players like to rock out with wild abandon, which means they need a limiter to keep their signal in check, to avoid unwanted speaker distortion.
As far as the various tone and envelope changes for flavor or fatness, they are completely subjective. Either you want those changes or you don't, and that's fine either way. Sometimes that rubbery thump, or boosted harmonics, are exactly what the song needed! And one compressor may be good for getting the tone/envelope changes you like, while another seems to do nothing but deaden your sound and add noise. That's normal. Read my article about good settings, and if that doesn't help then try a different compressor. But don't assume that because you have good dynamic control with your hands, or because you tried a pedal in 1985 that sucked, that compression is no good for anyone. It's a very useful and powerful (yet subtle) tool, but you must pick the right tool for the job and learn how to use it properly. I don't think compression is for everyone all the time, but I do think a professional musician should have the ability to use a compressor well, as part of their tool box of skills.