I travel a lot for work, which keeps me off the bike at times. Because of this I try to run 2 days a week, ride 4. On my travel weeks, my riding is cut down to 2 days but I can still get in a couple of days of running.

Being able to run in my particular case helps my bike performance through fitness maintenance and keeping the weight down.

Also, I believe the combo of running and interval training really help shed weight - which equated to better fitness on the bike.

Lastly, running seems to help work out kinks in my core that are otherwise stiff from riding.

Oh one last thing, really not a fan of running at all. :-(