I think (and I'm definitely outside my comfort zone here
) that self-connectivity is what allows those unexpected u-turns you see during navigation. In other words you are driving up a segment and Waze tells you to come straight back down the segment at the next junction. I presume that the segment is "connected to itself" and will therefore route back at the end.
Reverse connectivity (purple highlight) I have only seen with one-way segments, where another segment joining the one-way segment at it's "outlet" shows a purple highlight on the one-way segment. I presume this means that, even though it's one way in one direction, Waze sees a valid routing going the other way.
Mind you it is nearly 3am here and I'm watching Sunday Night Football and doing some work stuff and I really should be sleeping! I'm sure someone who knows what they're talking about will come along and correct me.