When I came into RPG gaming, the default ideal was that the GM customized the system and created the setting to suit the group. I was 21 in 1978, with a long history of playing wargames. I was used to modding and kitbashing, though those terms may be later than the practice. I bought blank counters and created my own units, re-wrote rulesets, and created my own maps on big sheets of matting bought at art stores. Coming into RPGs, I did the same thing. Before I ever played an RPG game, I was going through the ruleset, discarding, adding, and modding. I played exactly one game before forming my own group as GM, because I knew that was the part I wanted to play.
Somewhere in those long years, things changed drastically. Using commercially produced settings and adventures became the default. I was going along in the old way, and only noticed it when new players joined the group. I bought a few adventures - modules people called 'em for no reason I could see, as they weren't modular at all - and maybe used a bit or two from them. I bought the old Greyhawk campaign, and never used it at all. More and more games were coming out with default settings as opposed to the old idea of implied setting. Meanwhile my own games were moving very far indeed from the games as depicted in the rulebooks. I added rules from this or that game, changed character generation completely, dropped lots of rules that were cumbersome for me in play. The results would not be recognizable to anyone who played those games in any other group.
Currently, there is a fetish about playing games in the RAW - that is, Rules As Written. Changing rules, kitbashing, modding are all verboten. This is, in my opinion, just wrong. It's an abdication of the rights of the group to the rights of the designer, even if the designer intends nothing of the sort. It's passive - insidiously passive. I don't like it, and think it's bad for the hobby. Groups should be pushing their own agendas, and so should individuals. Leaving everything on the designer level, as RAW does, turns groups from participants into consumers.
The only time I can see as justification for RAW is in playtest, and even then the playtesters, when they meet with problems, should be willing and able to get around the problem in play before feeding back the problem and their own solution they used in play.
Anyway, as always, my opinion.