Usually when candidates announce they're "suspending" their campaign, that's a legal term of art that simply means, "I'm no longer running but I need to be able to still raise money to pay off my campaign debts." In Marco Rubio's case, though, maybe he is only suspending it. In this election year there's hardly anyone who can truly be said to be out of the running. And if we do get to a contested convention and the party starts looking for a compromise candidate, I suppose you'd be in a good position if you already have at least some pledged delegates.
But more likely, the request Rubio made today is designed to help deny Donald Trump a first-ballot majority: