Having the option to serve and volley gives you more of a capacity to make things happen from more areas of the court. Even if you can't realistically S&V all day against every opponent you face, it's a solid option to have in your bag of tricks and if you ever get into heavy doubles, you won't be able to hang if you can't do it well.
I usually prefer to go in behind a funky spin serve. If I'm not even sure how it's going to bounce, it will usually keep a receiver off balance and give me a little more time to set myself up a little closer to the net. My flat serves are better for just occasionally keeping someone honest. Instead of going in behind a flat heater, I might wait and see if it coaxes out a weak return that's short. Then I can move in behind my second shot.