They share the flight number mostly for marketing reasons to make the flights come up more prominently in the display.

As to whether a late inbound on segment 1 results in a delay for segment 2 using separate aircraft: in my experience they may hold segment 2 a little bit if there are a substantial number of passengers connecting from segment 1 to 2. But if they delay is long or few passengers are affected, segment 2 takes off on time and the late inbound passengers misconnect. I'd estimate that if you care, you have a slightly better chance of the second flight being held for you on one flight number compared to another similarly timed inbound flight (if only because the same flight number tends to make people book on it). It is also possible for them to hold international departures for other arriving domestic flights, if a substantial number of people are involved and the delay is not too long (SFO has done this on flights I've taken, generally for up to 15 minutes). In the reverse direction (international to domestic), I don't think it's normal policy to hold a domestic connection.

I think they often assign the extra letter without knowing whether the two flights will be in the air at the same time. I also think I've heard them attach the letter to the domestic segment even when that is the first segment and not to the international segment.