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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.

So is there some in or out of universe answer to why the Hulk was saved?

• Dark Magic, statistically the least safest way to travel

According to ScreenRant, Dark Magic is a dangerous way to travel. In the Thor: The Dark World Prelude Comic it is revealed:

The All-Father reveals that there is a heavy price to pay for wielding dark magic....The price is not only paid by the one who summons the Bifrost using dark magic; it is also paid by the one who travels through it. The experience of traveling through the Bifrost in this way is enough to leave even the mighty Thor crying out in pain, and he is rendered unconscious upon his arrival on Earth. The Asgardians actually fear the journey has killed the Thunder God, and their fears are only calmed when Heimdall finally confirms he has awoken safely on Earth.

The Hulk is more expendable than Thor, so he can risk the trip, which is hard enough on The Hulk that it turns him back to his human form.

• Hulk / Banner is an Earthling

The Hulk is from Earth, and might be a better messenger of destruction than Thor (who only lived there a couple of years).

• Thor needed a weapon.

Heimdall, the seer of seers, prognosticator of prognosticators, may have realized Thor couldn't go to Earth, because he needed to build a weapon that could beat Thanos. Specifically, Heartbreaker, er Dream maker, I mean Stormbreaker. Therefore he needed to go Nidavellir.

He also predicted an early spring [citation needed].

Heimdall sent Hulk because he is a better choice in the following ways,

1. He knows the Avengers better

He has worked with the Avengers for a longer period of time than Thor and he knows the Avengers better.

2. Heimdall didn't knew Banner couldn't transform

Heimdall has no way of knowing that Banner will be unable to transform into Hulk. Hulk is as good as Thor when it comes to fight. Also the weaponless Thor did not put up a good fight against Thanos as Hulk did. So Heimdall might have thought that Hulk would be more useful.

3. Thor had a chance of surviving

The life of Hulk was in more danger than that of Thor. No one was trying to end Thor's life the next second, but Hulk will be killed soon as we see Thanos approaching him after the fight was over.

Also we saw that Thor was able to survive in the space. Maybe Heimdall knew that and he sent Hulk because Hulk cannot survive in space.

And to address your points,

he was sent back to warn everyone, he could have sent Thor back to
warn everyone else.

He was able to convince Tony to call Steve Rogers. Tony took out the flip phone from Steve which was later used by Banner to warn about Vision. I seriously doubt that Thor would have convinced Tony.

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

Banner warned Strange early enough and was able to get Iron Man to the sanctum. Dr. Strange will have seen this after the Black order came but Tony Stark will not be there and he might not have went to Titan.

Also the protection spell around the time stone might have been placed by strange after the warning from Banner as there was no such protection in the Dr.Strange movie.

Banner also warned Cap about Vision. Otherwise they would have got the mind stone without a battle.