Translation memories and termbases, what's the reason for using more than one?

I'm very new to translating in any way other than on bits of paper, and I'm just wondering about the uses of TMs and termbases. All the videos I have seen make it look like translators use lots of different ones depending on what they're translating, but I can't work out why this is. Is it possible/advisable to just have one giant TM that includes everything you have ever done? If not, why not?

Thanks!