From a WaPo article:

Ironically, the first key for Rubio in Tuesday’s balloting involves cheering for a Trump victory over Cruz in Texas, which would embarrass the freshman senator in his home state and block his path forward.

This makes Texas a state where Donald Trump can’t lose in the long run, no matter what the outcome.

If Trump actually wins Texas, he picks up a lot of delegates. Even though Texas isn’t a winner-take-all state, there are still a lot of delegates, and the guy with the highest vote total picks up more of them than anyone else.

A win in Texas probably means that the turnout for Trump was actually higher across all of the Super Tuesday states than currently predicted by the polls. A resounding sweep of Super Tuesday, picking up lots of delegates, will make Trump unstoppable even if Cruz drops out the next day because he couldn’t even win his home state.

Rubio can dream that he can finally start winning if only Cruz drops out, but it won’t turn out that way in reality. Not after Trump gains massive social proof from such a resounding win on Super Tuesday. Current polls show Rubio losing to Trump in a head-on contest, even in Rubio’s home state of Florida.

On the other hand, if Cruz wins Texas (as currently predicted by polls) and Cruz stays in the race, then the anti-Trump vote continues to be split between Cruz and Rubio, and Trump should have the same outcome in future primaries as he does in Super Tuesday, which is that Trump wins at least a plurality of the votes and, with many future primaries being winner-take-all, Trump coasts to the convention with the majority of the delegates.