What does "if you can't beat them, join them." mean?

It means that if you don't have the means (ability or resources) to overtake a certain group, you're better off joining them so that you can still benefit. trying to surpass them might not end well for you, because they're more skilled or have more resources, so you decide to join their ranks to learn from them or share their resources.

I think the answer will be: I was hungry so I ate.