Instance does affect moderation policy though. For example, someone who likes the vibe of specific "controversial" instances like hexbear would likely find many instances to be dissatisfactory because many instances are defederated with it. Someone may also have strong preferences about "free speech vs safe space" kind of moderation.
General-purpose medium-sized instance is probably "good enough" (especially if picking your first instance, with a willingness to migrate later if needed), but someone will feel more at home if their preferences more closely align with the instance they're on. For example, beehaw users generally seem to love being on there from what I've seen, and I think they would feel less at home on most other instances.
There's also value in making use of an instance's local feed in some cases (less so for most general-purpose instances though!). I participate in aussie.zone regularly, and at times it's a pain to use from a remote instance. For example, if I want to submit a new post, I have to manually check several loosely-related communities to make sure it hasn't already been posted to the instance (cross-posting intra-instance is not ideal imo). If I were a local instance user, I could just check the local feed, which most of the time I just end up doing anyway because it's faster than checking several communities manually. It would definitely be easier for me in that regard if I just migrated there or had started there to begin with.