It is for feedback. Imagine we have a magical ideal opamp with inputs having no polarity. It can do anything to have its inputs equal (zero difference). Now you put 1V to one of its inputs and connect the output to the other input as feedback. It is now going to equalize its inputs, and it achieved that by raising its output to 1V. Hmm, it is acting as a unity gain amplifier or a buffer.

Now, instead of connecting the output to the input straight, you put a 9:1 voltage divider between them. Now it is going to try equalizing its inputs once again. To get 1V from this 9:1 divider, you need to supply it with 10V. Oh look at that, the opamp is going to do the same. It achieved equality in its inputs by raising its output to 10V. As you already guessed, you made an amplifier with a gain of 10. This is why there is a voltage divider in some opamp circuits. They can be used to divide the output too (gain less than 1) instead of multiplication, with a different configuration.

I don't know what happens if you don't let it equalize its inputs, it may act as a comparator or an analog schmitt trigger. I am a self-taught electronics hobbyist, I don't know much.

The feedback was negative in this example.