What are the round things near the main front cog? This bike in this picture has two rollers near the front sprockets. The chain goes through the lower one and not the top one. I've no idea on their purpose vs other bikes that don't have them and notice that they make a lot of noise. 

Questions


What are they, what are they typically used for?
Are they needed?
What would happen if if remove them?


Please refer to the image below
They are chain keepers.

Back in the day before narrow-wide chain and 1x transmissions, the front mech/derailleur was a great way to keep the chain on the chainring.  And given bikes had two or three chainrings the front mech was necessary for changing gears.

However MTB riders liked to bounce around a lot, and that could still "throw the chain" which removed your propulsion.  This was not ideal.  The two rollers pictured work more like guides than like jockey wheels.   They are there to guide the chain, and may or may not have teeth.

These would be more common on bikes with full suspension, where the chain tension changes as the shock compresses.  Your bike appears to be a hard tail, so only chain slap from bouncing or landing can cause a derailment.

Given this bike has a derailleur protector and relatively small rims, I think its a kids bike and the guides are more about protecting the bike from mistreatment rather than keeping the chain on during technical sections.

As for "chain goes over bottom roller and clear of the top roller" its because the plate in the back is maladjusted and needs to be rotated a few degrees anti-clockwise as pictured.  That it still rides okay as shown proves that this tensioner is relatively unnecessary.  You might be able to tap it into position using some gentle percussive maintenance, though this may upset the bottom bracket.   

If you remove the two guide wheels it would leave the chain a bit long, so two or perhaps four links might need removing.  If you remove teh whole plate, that will move the Drive side bearing cup inboard and possibly change the chainline.  I'd leave it in place personally once its adjusted.  

They make a lot of noise because the bearings are probably dry.  So remove, service and reinstall would be ideal.