Now this is an unusual scenario, but the closest parallel that I can think of is the Krogans of Mass Effect. It's complex, but the whole gist of things is that the rest of the alien races weren't strong enough to fight the Rachni (bugs from Starship Troopers). Why?

Salarians

These guys are extremely intelligent, amphibian people who have made large leaps and advances in the natural sciences and biology. Unfortunately, they are physically squishy, have short lifespans and aren't really that great at waging war since most of their people are intellectual types. Your PHD in neurobiology and nice guns isn't going to help you survive harsh conditions and grievous injuries.

Asari

Space elves with long lifespans that have powerful biotic powers, but also have an incredibly low reproduction rate and a culture thats more focused on the arts and humanities. Magical powers are cool, but the lack of heavy ordinance and a large regular army stops them from fighting a conventional conflict.

Turians

Collectivist bird like warriors that invest most of their resources into waging war. Their soldiers are mobile, relatively durable and possess the resources to field support vehicles. They're about on par with humans, so most of their might comes down to tactics and technology as well. Unfortunately, they still don't have the physical durability to wage an extended war on ground against a horde of space insects.

Krogans

A technologically primitive but physically strong race of alligator like reptiles. These guys can take large amounts of damage, they're extremely strong and can carry a lot of ordinance, and they're aggressive. All they need is some alien tech to get in the fight against those nasty space bugs.

Why would aliens need humans?

Maybe those aliens have some physical limitations that stops them from fighting a prolonged conflict. Being technologically advanced does not ensure that they are also physically fit to fight a war. Every conflict is won by putting boots on the ground.

Now this is an unusual scenario, but the closest parallel that I can think of is the Krogans of Mass Effect. It's complex, but the whole gist of things is that the rest of the alien races weren't strong enough to fight the Rachni (bugs from Starship Troopers). Why?

Salarians

These guys are extremely intelligent, amphibian people who have made large leaps and advances in the natural sciences and biology. Unfortunately, they are physically squishy, have short lifespans and aren't really that great at waging war since most of their people are intellectual types. Your PHD in neurobiology and nice guns isn't going to help you survive harsh conditions and grievous injuries.

Asari

Space elves with long lifespans that have powerful biotic powers, but also have an incredibly low reproduction rate and a culture that's more focused on the arts and humanities. Magical powers are cool, but the lack of heavy ordinance and a large regular army stops them from fighting a conventional conflict.

Turians

Collectivist bird like warriors that invest most of their resources into waging war. Their soldiers are mobile, relatively durable and possess the resources to field support vehicles. They're about on par with humans, so most of their might comes down to tactics and technology as well. Unfortunately, they still don't have the physical durability to wage an extended war on ground against a horde of space insects.

Krogans

A technologically primitive but physically strong race of alligator like reptiles. These guys can take large amounts of damage, they're extremely strong and can carry a lot of ordinance, and they're aggressive. All they need is some alien tech to get in the fight against those nasty space bugs.

Why would aliens need humans?

Maybe those aliens have some physical limitations that stops them from fighting a prolonged conflict. Being technologically advanced does not ensure that they are also physically fit to fight a war. Every conflict is won by putting boots on the ground.