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 have to be prepared for every possibility. In the case of an unforeseen circumstance and, for one reason or another, you have to play on grass rather than synthetic turf, what will you do if you're unprepared?
And while this isn't news to the football world, you need shin guards (yes, they're required), a pair of low socks to wear underneath your long socks during matches, a pair of low or high socks for training sessions, shorts, a jersey and don't forget your shoes in the locker room, it's happened to a few (yes, a few) of my team-mates and it's particularly bad luck.