The classic 2" (5cm) Speedo has a smooth front.
When a swim bikini briefer than this is wanted there can be a shortage of space at the front, sewing 2 pieces of fabric together along a convex curve creates a bowl shape which when incorporated into a swim bikini becomes a pouch.
When a Lycra swim brief ages the first thing to go is the rear, sagging down towards the knees. The Paper Lycra suits of the late 1980's were worn very tight but were prone to rapid aging and rear sagging. On stretch fabric a straight seam (sewn with the fabrics stretched) gives stretch down the seam but not across it. This can be used to give support to the rears of rapidly aging fabrics so the Paper Lycra racing speedos had rear seams.
The rear seam was found to be attractive and the Japanese incorporated a narrow piece of mesh into their rear seams to aid drainage. This feature was also popular with swimmers.
In conclusion, a front seam creates a pouch while a rear seam makes the suit last longer and give the option of adding drainage mesh.
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