Its because the runner could bump into you

It’s pretty much what Will said, but more specifically it’s so that you don’t get your ankle accidentally stepped on. It’s just to make sure you’re safe. I will say, however, that bad umpires might call you off the base, so you might not want to do it if you know the umpire sucks. Otherwise do it every time you can, especially with other runners on base so you are ready to throw the ball.

Different foot touching the base style was taught by the Orioles and the Dodgers at one time.