I'm a roadie mountainbiker. I started swimming again about 18 months ago after a gap of 10 years (I used to swim competitively as a teenager).
I've found it's helped with fitness. I don't know the technical terms but I have better control over my breathing and can climb faster while not coughing up a lung I also recover faster after going anaerobic.
I swim about an hour a week doing - sprint/technique work (interval type stuff) or pure endurance (normally try for 2 miles in the hour).
Other benefits are that it's different from cycling so I can still go if I've done a hard ride and in winter it's nice and easy to go after work whereas a couple of hours in freezing rain is less appealing.
Oh, and girls in bikinis.