Rankings mean more or less depending on the stakeholder. You all are somewhat correct. Do NBA scouts care about where a kid was ranked coming out of HS? No. Not at all. But a 17 year old puts a lot of stake into where they are ranked as it's a status symbol of sorts. Also, they put a lot of value, probably too much, on rankings in who they are going to play with depending on what they are looking for. Too many highly rated recruits might mean one kid wants to go somewhere else to get more time. Another kid may see that as an opportunity to compete at the highest level and be on a team that will win a lot.
As far as coaches, it probably matters less than what people think. They want winning teams and you can build them in many ways. Role players are valuable, ranking scouts don't always get it right, there are late bloomers, and there is a limit to who you can get sometimes geographically. However, the coaches also need to appeal to the mind of the 17 year old. The rankings surely get brought up when discussing who they are going to be playing with. And when you are pushing to get a top 20 5 star kid it always nice to have a resume of having secured and developed top 20 5 star talent.
As far as fanbase, it seems most care because it is something to brag about but there are plenty of older wiser fans out there that know that a 75th or so ranked recruit like Bouk for instance could be like landing a 5 star. And conversely a 10-15 or so ranked recruit could have major bust potential. Ajou Deng rings a bell. So although its great to laud a top 5 rated class, the wise ones know that doesn't mean much if you don't develop/manage/put your players in positions to succeed because then the narrative will quickly shift to being a coach who wastes top end talent.