IPCs in the bank don’t win the game; units on the board do. Every turn you wait to purchase a unit that you can afford is one less turn that unit will be on the board, one more turn it will take to get it where you want it to go, and one less turn it can spend taking territories to get you more IPCs. It’s like the time value of money, a dollar today is worth more than the promise of a dollar tomorrow. A piece of armor this turn is worth more than that same piece of armor next turn.
Every time you save IPCs, you’re sacrificing valuable initiative. More time in the bank means less time on the board. If you’re going to sacrifice that initiative, I can only think of two good reasons to do so:
You are saving up for something that costs more than the IPCs you have on hand, and whatever you intend to buy with those IPCs is worth the cost in initiative; or
The value of keeping your opponent guessing what you’re going to buy is so great that it outweighs the sacrifice in initiative. This is likely to work best if your industrial complexes are close to the front - the farther your industry is from the front, the worse this strategy becomes, since you’re essentially making getting your units into action take longer.