I would not set your training goals around average speed. However, average speed can give you some useful information when taken into account with the rest of the ride conditions.

For example, when I am doing one of my large team rides, if I see that our average was 20mph or more on the way back into town, I might think, wow, this ride was really "race pace" (for me, maybe not for most of the guys). Of course, this is in a large group, and I will never put myself into the wind unless I need to bridge a gap or go for the town line sprint. On the other hand, if I do a ride on similar terrain by myself or in a small group and the average is 16mph or less, I may see it as a good aerobic, endurance ride if we were rolling smoothly. Or, it could be poor training for me if I am leading new cyclists, for example, and we are stopping or regrouping a lot. A 16mph average or less could also mean a really good, really hard ride if I'm out in the mountains where I spend a lot of time sub-10mph but at or above threshold.