I bought some leggins for running, cycling, stretching… for winter (sale) and summer.
The summer ones have some translucent bits on the back side of both knees.
There are other summer leggings, completely closed from the same brand that I didn’t buy because to me those bits are well placed to get rid of moisture, and are more expensive than the completely closed ones. Maybe the seller used my naivete to convince me to buy the ones with translucent bits?
So, fashion or function? because to me the completely closed ones are more fashionable.
The closed ones are cheaper than the ones with bits, so I assume the ones with bits are better
Based on things I own, cycling leggings and motorcycle pants both tend to have wind proof, water proof front panels and at least some breathable rear panels to allow for, as you mentioned, getting rid of moisture.
And last time I bought cycling shorts, basically the top part of leggings I suppose, the more expensive ones were made from more panels allowing them to fit and flex better.