Well, to be technical, you flip a coin, or spin the racket to get
the choice to decide between serve and side.
Sometimes if the other player has a weak serve, you want
them to serve first. Likewise, if the sun is bearing down on
one side, you may want to start off on the other side
regardless of serve.
Every tournament I ever played in, we always spun the racket to
decide who gets to choose serve or side first.