I'll build an example that might help get my point across, which I may not have explained sufficiently.
So imagine a character concept of an older "wizard" who is actually a scholar with not a whiff of magical talent, who "made a deal with the devil", gaining arcane power in the process. So tome fiend warlock.
The mechanical build is a blaster - EB+AB, fireball, that kind of stuff.
But here is where who chose the spell "in game" (the warlock or the patron) becomes important. If the scholar is the one choosing their spells and they are taking blasting spells, it shows that this is what the scholar wants - to have the power to blow stuff up (vs more finess/utility magic).
However, if the patron is the one who chose the spells, then perhaps the relationship between the scholar and their magical powers becomes fraught. The scholar is dismayed, even afraid, of their own powers. Perhaps the scholar wanted tongues and hypnotic patterns, but instead got fireball and summon shadowspawn.
I mean you could say that the patron and the scholar both want to blow stuff up, and in that case it doesn't matter who chose. But if the patron is the one making those choices, it opens up a different roleplaying direction.