Like elephants, they do two things very well: 1) disrupt enemy formations, 2) scare the heck out of enemy formations. Neither does a whole heck of a lot to kill many soldiers on their own.

However, if you happen to have a unit of cavalry right behind them, or have and advancing line of soldiers, this can often be enough to either rout them, or put them at a severe disadvantage.

Chariots (in RTW) are best when used in a combined arms setup. Other units can accomplish the same thing. I prefer elephants when I want to disrupt a unit before cavalry hit them, because it's much more believable that your units don't get injured with elephants than chariots (elephants can actively avoid your guys, wheel spikes aren't as bright or as discriminating).

Chariot Archers make good mobile arrow towers (arrow towers like those on stone walls, for example).

Since you're playing the Seleucids, I'd recommend focusing on Elephants instead. They do the same things, tend to kill faster, and aren't nearly as frustrating as Chariots (too many times have I had chariots unable to figure out how to go through a gate). However, building one or two units of Chariots to try out might be fun. Keep in mind that Chariots also can go amok, and there's no spike to kill them with if they choose to go amok with your soldiers ...