As webcomics progress, they eventually get complex enough that a new reader may stumble onto the latest strip and not know what's going on. So the time is right to add a New Reader Guide - complete with overview about what the strip's about and profiles for the cast.
Why is it "the right time"? Because I think it's better to let the strip itself show what the characters are like, instead of decreeing it up front before it gets rolling. Also, the profiles may change as time goes on and the stories develop - so telling too much in a profile would spoil it. Anyway, I can at last know that new readers can come into the strip and get up to speed.