First reason is profit. Companies want to emulate the success of those shows in other markets. The second reason is to try and domesticate the show to attempt to get a broader appeal in those new markets. Even from a British > US version point of view, it's happened to a significant amount of shows, I think the success rarely translates however.
Best examples I can recall are The Office and Gordon Ramsay's various shows, however even those have some significant variances from the original works to increase success (i.e. Michael Scott's change in personality/appearance, Gordon Ramsay's US shows being highly dramatised and editted).
On the other hand, US versions of shows like Dad's Army, Red Dwarf, Inbetweeners heavily flopped despite major success in the UK. Wether that's down to those shows just never realistically having a chance in the US market or just pure execution, it's difficult to tell.