It's not hardly essential if you have a decent bass. There are three reasons I can think of to use a compressor:
1. The biggest reason compressors are used is to get more sustain out of your bass. My bass has superb sustain, so I don't use one. Using a compressor "most" folks set the ration to 4:1. That setting means that for every 4 decibels of volume increase you will only get 1db of actual volume increase and the rest of the energy will go into sustain. 3 to 1 is also a common setting. You'll still get decent attack out of your playing but increase the sustain. The higher you dial up the ratio the less attack you hear and thus the less dynamics you'll have in your playing.
2. To get a specific sound or effect out of your bass. If you dial up the ratio a lot, you can almost make your bass sound like organ pedals - no attack to the note and sustain forever. There are a few places where that might be desirable and some folks want just a little attack and lots of sustain in everything they play. The compressor is how to get that.
3. To protect your speakers. This is actually the main reason that FOH systems use compression on your channel. They don't want any sudden spikes of power from you diggin'-in on your bass or playing really low notes to damage their speakers. This is not necessarily driven by making you sound better (it can be), but by making sure their speakers don't get fried.
My opinion is the old "if it ain't broke, don't fix it". If you have a problem with not enough sustain or if you are making your speakers fart when playing loud, then a compressor is a good choice.
Ideally you'll want a compressor that has 4 controls - Threshold (controls when the compressor activates), Ratio (controls how much of the signal is compressed into sustain), Attack (controls how quickly the compressor engages once the threshold is reached- often see toggle switches of "soft knee" or "hard knee" instead of a much more variable dial), and Decay or Release (controls how long the compressor keeps compressing before it lets go once it's clamped on).
At the very least, you'll want something that will give you threshold and ratio. Usually those compressors have an internal setting on the attack and decay. Some are good...some not so good. You really have to play with those to see what you like.
I don't use a compressor because I don't need one for sustain, my speakers aren't in any danger of being stressed, and I don't play much in the way of slap. Playing Slap usually needs a compressor because of the huge attack caused by hammering on and plucking up on the strings.
Hope that helps.
