Well, "optics" is a broad category, but taking up these reflex/RMR/this beast sights:

1. Some you have to switch on/off, some have motion-activated power-on, some have auto-off/timers.
2. "Better" is so subjective as to make any answer meaningless, but some folks find them to promote quicker acquisition than irons and to get a more reliable point of impact (if, for example, you have a front sight that has a comparatively large space between it and the notches of the rear sight, and have trouble centering it right/left or up/down, the red dot tends to eliminate that (i.e., tends to be more idiot-proof)). That said, they do have down-sides (bulk, battery dependence, there are others), so it's really down to personal preference.
3. You need a slide that is milled to accept them most of the time (some mounts fit into some rear sight dovetails, though that is not the most common method).