I am imagining the future. Suppose after trying SACDs for a while, you feel that it is not worth it. Eventually, your SACD player dies, and then at that time there will probably be very few SACD players available. So you might decide at that point to just buy a CD player instead. You still could play your hybrid SACD discs on an ordinary CD player (though only the CD layer), so you don't lose all of the music that you bought in that format.
The beauty of a hybrid SACD is that it is both a CD and an SACD, and therefore can play in anything that can play either format. Of course, if the player one uses can only play CDs and not SACDs, then one will only have access to the CD layer, but one still gets to hear the music in CD quality sound. It is not like you would be stuck with a bunch of discs that you could no longer play.
Frankly, if Philips and Sony had wanted SACD to take over, they should have, for the most part, stopped releasing ordinary CDs, and just released nearly everything as a hybrid SACD. That way, every disc of these that they sold would be an advertisement for the format, and many people would be sufficiently curious to try this new high resolution format, and many would want access to multichannel sound. But they did not do that, and so it has always been a small market.