why are the rails on a seat angled?

[700 x 23c]

In other words, why are then not parallel to the ground such that when you change your fore/aft position you don't have to then change your tilt to keep the seat level?

[CNote]

cos the seat tube is angled

[Dave]

Most saddles don't have much angle to the rails, but obviously (?) this will vary depending on the angle that the user choses for the saddle.

Most of mine appear to be up no more than about 5 degrees. Even 10 degrees will raise the saddle less than 2mm for each 1cm of forward travel.