Because the weapons become better as you upgrade them. Larger clip size, more ammo, less weight. I'm not doing it to unlock them all or upgrade them all, I am doing it to become more effective in matches. Cerberus Harrier 1 is great, but the clip size is very small and the weight doesn't help either. Getting it leveled up will make it a more effective weapon on my Turian Ghost. I'll be able to fire longer and use my Tactical Cloak + Overloard combo more often, since the power cooldown isn't affected so much.