The only reason any casino has a pending period is in the hope players will reverse their withdrawal and subsequently lose.
Because they've already been mentioned here, I'll say - 32Red stopped doing weekend payouts, but then realised they still weren't making enough money, so stopped manual flushing, but still decided it wasn't earning them enough and finally added a 24-hour pending period as well (I'm not sure what order those last 2 changes were made because I jumped ship long before that).
I can't imagine how much they've earned on the strength of "weak" players who still love the site but don't have the willpower to wait out the pending period, and often the entire weekend and even holidays as well.
I don't blame 32Red of course. They were the standard bearer for years, but the casino industry has reached a saturation point with countless browser-based multi-platform casinos, many of whom payout before 32Red have even looked at a player's withdrawal.
I'm sure that alone cost them a lot of customers, what with them being MG-only and well behind the times. So they had to make changes to cut costs and increase revenue. What better way than to implement an archaic payment system that sees the players do the cost-cutting for them? Saves them improving the casino and trying to move forward with the times.
That sort of thing would only cost them money, after all