There are dozens of certification agencies, some larger than others but essentially they all do the same thing: teach people how to dive. They provide one a plastic card that says so.
Now, many of mentioned agencies don't agrer with others on what is the "best" or "right" way to teach scuba, some are commercially oriented while others can be some non profit or whatever. Sometimes the differences or disagreements are minor, nuances, sometimes not.
But what is for sure, is that many of them have recognized a problem that we often see being questioned here: "I have a X certification, want to go diving vacation in country Y, will my X c-card be accepted even if the dive operation is affiliated with Z agency???"
Those are probably the majority of divers, the largest chunk of OW and AOW (or * and **, or whatever name the agency has invented) and I think of RSTC as some sort of council between various agencies to accept one the other's basic certifications so people won't be concerned with those when going diving in different places and finding some unfamiliar operators affiliated with some other agencies and not necessarily the ones with the logo on your card.
It probably makes the scuba business better for everyone involved, both the training agencies and us, customes/divers.
The minority of divers have more advanced training certifications, and many agencies think that the difference or gap is so large, that if you are you are, say, an instructor of agency X and want to work in a different location that certifies for Y, they may require some "cross over training".