Units I understand. Taking too many turns to build them while an army was on it's way towards you would be really bad. The war might be over before you've managed to build another unit at all. This is of course the same for the AI; it needs to be able to build up some defensive units if an army is headed for them.
Buildings I'm less clear about why though one low priority reason could have to do with units as well; Quite a few buildings are required for certain units, and even more buildings grant bonus promotions or XP to units. Taking too long to build those might mean taking too long to get an effective army, both for offense and defense.
The only thing I can think of is for game gratification. Taking forever to grow is one thing, at least with faster than normal buildings you feel like you are getting somewhere even if the cities aren't growing fast.
Cheers
Buildings I'm less clear about why though one low priority reason could have to do with units as well; Quite a few buildings are required for certain units, and even more buildings grant bonus promotions or XP to units. Taking too long to build those might mean taking too long to get an effective army, both for offense and defense.
The only thing I can think of is for game gratification. Taking forever to grow is one thing, at least with faster than normal buildings you feel like you are getting somewhere even if the cities aren't growing fast.
Cheers