I will repeat a couple of my earlier points
Many times you have limited capacity, like a minor IC.
Also limited are transports. The most potent load for a transport is infantry/tank
And don’t forget about damaged IC’s - when damaged, the cost of every unit goes up 1. Infantry costs 4, arty/mech cost 5, and tanks cost 7. Now the relative cost of tanks over cheaper land units is decreased.
But even in normal circumstances (like building in Berlin or Moscow), tank purchases can make sense because of what Zhukov and JDOW have said.
Of course Allweneed is right that tanks costing 6 instead of 5 is a significant nerf, but the tanks at 5 in earlier versions (with a defense value of 3) were overpowered