Having worked for an online casino I know why the biggest / real reason that match or welcome bonuses are used.
Due to the games random nature its like an insurance policy against a player getting lucky early on, as then they are forced into having to play a large amount of it back through and hence minimising the "risk" to the casino.
If the player don't hit anything big in that bonus period then they have not lost anything either and the player then falls into the "well at least half of it was their money" scenario and don't feel as bad hence player slightly happier than would have been even tho they have still lost.
Its all very much psychology at work.
For optimum play you are and always have been better off opting out of all bonuses, then if and when you hit it lucky you can cash out anytime.