Resting for brownie batter
No, there is no benefit from resting brownies, and there would be a couple of small disadvantages to doing it.

Time will decrease the effect of baking powder since its work begins as soon as you add moisture.

Time will also cause gluten to develop in the flour, which isn't something you want in brownies. Gluten development will cause brownies to be tough instead of fudgy/chewy.

You aren't waiting for yeast to get frisky; you don't need or want gluten development; you're not shaping dough. You can and should just go straight to the oven after mixing the batter.