For some, it may be the only way (if hubby is truely a cuckold, or is in a FLM) they get enjoy their wife or GF. This is the most arousing orgasm a man can have, and a sexy wife can exploit it to great advantage - with an all too willing husband!
On the other hand....It doesn't need to be a "cuck" scenario at all - mfm threesomes are a great way to provide our wives with very satisfying sex and for us to enjoy seconds without threat to our marriage. I was lucky to find that my wife loved giving me seconds as much as I enjoyed being in her that way, and because hubby will be there the wife may feel more secure inviting another man into the marital bed.
Threesomes are an excellent way to move into "sharing" one another.