We all work better when we have something to clearly define ourselves against. An enemy or anti-us, so to speak.
An enemy gives us a clear definition of what we're not. A clear line of separation between us and them; similar to how the yellow lines in the streets define traffic flowing one direction versus the other.
I am not that. I will never be that. That is the exact opposite of what I represent. That is absolutely not me.
An enemy doesn't need to be a person; it can be an idea, a place, a way of being, or a state of mind.
Having an enemy assures you won't grow complacent. When you know what you're against, it's easier to figure out exactly what you are for.