It's not EGA: 16kB of RAM, 6845 CRTC, a character ROM and discrete logic means its got to be CGA. The second RCA jack could be for a light pen but those usually had a button on them and also required power - the RCA jack doesn't have enough lines for that.
So, I'm going to suggest the second jack is for a second composite output. And indeed there does seem to be a second DAC (2N3904 transistor and some resistors) way over on the other side of the board. There is a third 2N3904 as well with some unpopulated resistors - not sure what that's for (a third, internal composite connector maybe?). I'm guessing this extra feature was added as a marketing point and because it was cheap and easy to do rather than because it was particularly useful, but perhaps some people had applications for driving multiple composite monitors from a single CGA card such as public information display screens in airports and railway stations.