I have a low-end road bike with some upgrades that I've put most of my commuting miles on and have also used for triathlons and other events. It's almost 10 years old now. I suppose if I ever got really serious about triathlons I'd get a higher end bike for them and not use it for commuting except for maybe once in awhile.

I also have a winter bike since my road bike doesn't have enough clearance for tires I'd be comfortable using on ice and snow. And finally last summer (mostly on a whim), I salvaged my son's crashed 80's era road bike and converted it to a fixie. That became my main commuter bike so I'm not quite sure going forward what I'll end up using most often for commuting.

To your question, I don't think there's any particular reason you need a second bike for commuting unless your current bike falls short for that purpose in some way.