This is done in order to try and persuade punters to change their minds about withdrawing and actually bet their funds instead.
The way in which sites generally do this is by adding a period of time during which withdrawals are ‘pending’ before the transactions actually start to be processed. It is worth keeping in mind, however, that some gambling operators do further delay the provision of your hard-earned winnings. The above should give you all of the information you could possibly need regarding different withdrawal methods and their processing times. Do Some Bookmakers Deliberately Delay Payouts?