Why did the Thing alien even bother trying to assimilate the men?

We don't know it wanted to get back into outer space.

Maybe its survival strategy is just to take over as many organisms as possible. With only a few humans at McMurdo Station, its biological imperative was bound to go unsatisfied.

But the other posters have it right - it's an alien, so we don't know exactly what its priorities actually are.

Personally, I like to think that "The Thing," in its natural state, is just a single cell. Its sole imperative is to consume. It mimics solely to help it consume. Its mind is only as sophisticated as its current host.

I think its intentions were to consume the world. It went after the men thinking it would just wait for supplies to arrive and it would hitch a ride to civilization from there. But its ass got discovered.

I would argue that it felt that this was the best case for two reasons: assimilating the men spread itself around (aka biological urge to pro-create) and if the men were burning the imitations so quickly, wouldn't it have a better chance of winning if the men were preoccupied with the assimilations instead of instantly burning the main Thing and winning with a first round KO?

It's the Thing's nature to want to spread and take over other life forms.

The Thing had to look like a man to repair the ship. He was using parts from the helicopter. He could not go about this work in his natural form.

Just because it makes a saucer doesn't mean it wants to go to outer space. Just someplace else.

I think the Thing had a biological need to feed and assimilate. Without that it would go into a dormant state or something.

Now thinking of it, the Thing was stupid for assimilating all those dogs. It's what made the people suspicious. If it hadn't started assimilating all those dogs nobody would have gotten suspicious and tried finding ways to destroy it. It's almost like the Thing was just like, "I'm hungry and these other dogs look delicious. I don't care that it'll bring those other living beings directly here! I'm gonna eat cause I am so hungry!" Then it just does that bringing everyone running over there. If it hadn't done that and just assimilated the first person to check on the dogs the next morning, it would have been able to assimilate more people. But nope. It had to eat a bunch of dogs because it could only think with its stomach.