Why is pressing = repeatedly on iPhone calculator useful?

It's an auto-constant
Whether or not you find it useful depends on what you want to do.

Your method of just hitting = over & over just repeats the last calculation, so
2 * 2 = 4 = 8 = 16 = 32 etc

But, say you needed to calculate tax on a set of payments at 15%

If you want just the tax, type
100 * 0.15 =
then your amounts
87.20 = 13.08
43.57 = 6.55
12.50 = 1.88 etc..
If you wanted the amount including tax, you'd start with
100 * 1.15 = & off you go with your amounts, as above...

From comments - this function has been on calculators [in slightly different ways] since at least the mid-70s. It's not a new feature.
On the early ones, 2*=======... would quickly just ERR. Since then they've got somewhat more flexible & move to scientific notation.