You really want one of these on most ships that you launch (exceptions to every rule, and so on). It allows you to do launches without having to constantly correct your ship's trajectory.
ASAS will send commands to aerodynamic control surfaces, thrust vectoring engines, RCS, and yes, even SAS modules in order to keep the ship pointed in a given direction. In short, they're practically useless.ĪSAS, on the other hand, is incredibly useful. Finally, SAS does nothing for you in terms of docking craft together, especially since you're already going to have RCS. During launch, you're better served by thrust vectoring and aerodynamic surfaces than SAS, and in orbit, you should have more than enough time to orient even large vessels with just the torque provided by the command pod, with maybe a little help from RCS. You'll find that many KSP YouTubers don't either, since there really isn't that much of a benefit to them. Having said that, I don't use SAS modules except in extreme cases. This is beneficial for reducing roll during launch (if you don't use aerodynamic control surfaces), and for pitching and yawing a large spacecraft once in space without using RCS or thrust vectoring engines. The only thing that SAS modules do is increase the amount of torque available for rotation. You need an ASAS to command the SAS modules in order to have some form of auto-pilot.
SAS modules, by themselves, will do nothing to keep your ship pointing up.