I've heard players say - and have said it myself, once or twice - when rolling up a character and the first five rolls have come up somewhere between good and stupendous: "Come on, dice - I need a '7' to make this playable!". I always find that if I don't have an idea for a character concept going in that rolling something like 14-14-12-12-10-10 or even 18-15-14-14-13-13 is less than no help at all but give me 18-15-14-13-10-7 and I can give you memorable entertaining characters until your dice wear down to plastic marbles.
Nail hit firmly on the head...
I rolled 5-8-9-12-15-16 for my current character, and from that came my young, fragile Tiefling Bard, wonderfully likeable but very gullible and possessing no common-sense. I wanted to play a good natured but rebellious character so I therefore gave her the criminal background and decided she'd been unwittingly mixed up with the 'wrong' type of people while believing she was fighting for the oppressed underclasses - due to treatment she got when she was a younger because of her heritage.
That wouldn't work as well with point buy - a Wisdom of 8 is not low enough. And no, I won't disconnect her personality from her stats to force her to be 'less-wise', I can't do that, it feels wrong. Just as I won't try to portray a character with 9 strength as 'he-man', or 9 charisma as being a top diplomat.
Another problem with having a class/background/personality set in stone before beginning is 'what if that does not fit with the rest of the party, or the world presented by the DM'?
Our session 0 tends to go as follows:
- DM describes a bit about the world, some expectations, and what classes/races are available
- we all roll dice together to generate stats, and talk about what we'd like to play - TOGETHER - this is a group activity
- if anyone wants to tweak anything about the class/race to fit an idea they have, the discussion is done now - everyone gets involved, but the DM has the final say.
- we go away and come up with short backgrounds to fit in with what we discussed in session 0
This way we are all on the same page! We're a team, and the game is more fun.
If someone turned up with everything already done, based on their own assumptions, isn't that a little selfish? It's like 'I don't care about the rest of you, I'm doing it like this!'
If I'm DMing I'd nope that right away.