Let's start at the beginning: you'll need at least two pairs of boots in your bag - one for dry pitches and one for soft pitches. You need to be ready for every eventuality. The unexpected can always happen, and if for some reason you end up having to play on grass when you thought you were playing on an artificial pitch, what are you going to do if you haven't prepared for something like that happening?
I'm not telling you something you don't know when I say you need your shinpads (yes, you have to wear them), a pair of short socks to wear under your match socks, a pair of training socks (short or long), shorts, a shirt and, last but not least, your boots. Some of my team-mates (yes, my team-mates) have forgotten to bring them in the past and it's pretty inconvenient.