The cyclo allows me to track average speed on all rides, solo or group. That's how I can tell what speed group I fall into.
On group rides I like to see how fast the group is going. Sometimes the folks doing the pull are going too fast and string out the group. If I know my speed I can tell if they're pacing properly or pulling too hard and as second in line, I'm supposed to pull them back. When I'm pulling I need to know if I'm going too fast or too slow. On one ride I was pumping my brains out, fighting for speed. I was sure I was pulling at about 20 MPH or more. When I looked down, my speed was 15.6MPH. Time to fall to the left and let someone else take the pull, I had hit the wall but didn't know it.
This year my objective is to move into an "A" ride group. The only way I'll achieve that is to start training, like now, and keep track of my average speed, ride time and distance. I find my estimates of average speed are rarely on target. The cyclo computer is the best way to see if I'm improving.
EDIT: As for aero bars, my neck is already missing two discs so riding in an agressive position is difficult, I don't think aero bars are an option. However, I intend to try them just to see if they add any value.