If you throw 95 MPH in a barn in Eastern Siberia someone is going to hear about it whether or not you went to a showcase.
My most "celebrated" HS client (whatever that means) was given free entry to PG events because they needed him to make the event look less embarrassing. He throws 90-92 which is all that really matters but his pitchability is also a plus trait. Unsurprisingly he's a PG "10."
He went to one event throwing 82-84, same stuff, same body, same great pitchability. OK breaking ball, plus changeup, outstanding command. No one gave a damn. He went to one six months later and hit 93 and everyone was tripping over their clipboards to offer him a scholarship. Not a damn thing changed besides his velocity.
Trevor Bauer never did a single showcase except for a walk-on one he did for free because he wanted a few more innings on his buddy's team. What he did as a JR in HS was throw 94 MPH with plus secondary pitches.
If velocity didn't matter, Stetson Allie (OH) would have been a late round flyer at BEST.
Did you get a good look at that? Here, I made it easier for you.
Allie hit double digit batters in HS and walked every other batter, doing more of the same on PG's circuit of events. But Allie could hit 100 MPH and reliably sat in the high 90's, so who cares?
You'll note on that PG page he was drafted in the 2nd round and was paid 1st round money.
If you throw 86-88, these events might make sense for you if you have projectability (height) and great command. If you throw 90+, these events are largely irrelevant. If you throw below 86, these events are also irrelevant.
What everyone knows but no one wants to admit is that throwing hard is what matters the most to get the most amateur looks out of HS. College is a different story, but not much of one.