Well, topspin second serves, for sure, you gotta hit upwards so the ball clears the net. If you hit straight out level with the ground, the topspin brings the ball down below net level.
For flat serves, you don't need to clear the net by more than 10", so while your ball CAN leave your racket dead level with the ground, a racket moving upwards with a SLIGHTLY closed face can give you a deadball effect, like a knuckle ball, so the ball arcs downwards, but you hit a flat fast serve.
That is an advanced technique. If you do the math, your strikepoint maybe mid 9' above the net, you cannot "SEE" the service court, and neither can the ball. You need to impart some curve to the ball, but not topspin, as that slows down the speed of the ball.