This last photo is of the old, blue suit on top of the new white suit. You can see how different the sides are

Why do people cut out the lining? Is there a reason for that?
I also see people wearing 2 swimsuits, one on top of the other. Is there a reason for that also?

answers
 

1. Better body definition (and can let you swim faster according to some - but better make sure the fit is good as loss of shape in some materials can make it a mistake)

2. Swimmers might do that in training for a drag effect and so make them work harder on their lap times. Less so now but formerly common practise in water polo as a defence against aggressive underwater grab and pull tactics.
(somewhere on the www is a pic of a water polo international emerging from the pool with no bum cheek cover left on his speedo!)