I knew a guy from the Dominican Republic in high school who bred and fought bettas when he lived there; apparently breeding quality fighting fish, putting them up against another person's fish, and betting on the outcome was a fairly common practice. Can't remember the details, but 'feral' B. splendens would apparently be caught out of ditches and ponds, and be line bred for certain traits that were more desirable- vivid reds and dark greens were emphasized in terms of colour, shorter fins and bulkier body were in higher regard, fish with greater aggression would be chosen as future breeders, etc. Apparently males would be "trained" to react to a rival betta with a black plastic bag wrapped around something, like a finger. Fish would be placed in a fairly small tub for the fight, and the first fish to submit by turning pale would be the loser.
I don't really know what the point was; seems more like it was something done out of tradition? I do agree that the practice seems rather senseless and cruel; unfortunately I have no clue whether it still occurs on a regular basis today.