This question already has an answer here:
In the opening scene, Heimdall sends Hulk back to earth. But I can't understand why he would do this over sending Thor back to earth or anyone else. He cares more about Thor than the Hulk. I am not asking WHY he sent Hulk back to earth, but why he sent him over anyone else.
Here are a a few reasons why it is not abundantly clear even after watching the whole movie as to why he did what he did.
- The Hulk himself does not make an appearance after that scene (Although Dr Banner does, he does not make much of a contribution. And some people might say he warned everyone that Thanos was coming, but I am sure that after Thanos's goons came to earth and tried to take the Time Stone that Dr Strange could have seen this)
- Dr Banner is pretty much rendered useless as an Avenger due to the Hulk not coming out. (That is a big reason he is in the Avengers, or that is what I was lead to believe)
- To address a comment that he was sent back to warn everyone, he could have sent Thor back to warn everyone else.
There are probably some other points, but what I am trying to get across is that in the movie and after it ends, it is still hard to see why the Hulk was saved. So, there is only really one thing to conclude from this and that the Hulk is going to play a big part in future films.