Veggie soup w onions - is it better to caramelize the onions before putting in pot?

Hi there! It’s not totally necessary, but caramelizing/browning your aromatics adds flavor, sweetness, depth, and color to the soup base. If you decide to do so, I’d recommend caramelizing/sautéing the onions (which you could do in the same pot before adding broth, stock, or water) with a splash of oil and a generous pinch of salt until they’re soft and translucent.

Yes the flavor is different.
If you just put the raw onions in with the liquid and other ingredients, you'll get an onion-flavored soup but the onions will be plain in flavor.
If you saute or caramelize the onions first in whatever fat you're use, then add the other ingredients, you will get a different and (many think) better flavor - the sauteeing will reduce or eliminate most of the water in the onions, add flavors of fat and caramel, concentrate them.
You choice, depending on time you have and your taste preference.