Why the swimbriefs has a small lining inside the front?

[midlifeswimmer]

The front liner is generally a less stretchy fabric that the body of the suit, so that it holds things a bit flatter, and makes the suit less "shape revealing."

It's only on very pale colored suits that the liner becomes a prevention of "see through."

Speedo USA has taken to full liners on quite a few of their suits these days, particularly Endurance Lite ones, and ones with light colors in the pattern. They're often black, which causes the suit to look a lot less colorful when wet. That's a downer in my opinion.