Why is pressing = repeatedly on iPhone calculator useful?

Press these keys on iPhone calculator:

5 + 5 =

Unsurprisingly, the answer is 10. But keep on pressing the = sign and this is the output:

10, 15, 20, 25, 30, 35, 40, 45 … and so on

Why is this 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 =`
`87.20 =` 13.08
`43.57 =` 6.55
`12.50 =` 1.88 etc..
`100 * 1.15 =` & off you go with your amounts, as above...
On the early ones, `2*=======`... would quickly just `ERR`. Since then they've got somewhat more flexible & move to scientific notation.
• For sure, it's been there a long time, my 70's LED Texas did it - though surprisingly not on all & not all work the same way round. I've known my tax example to need `0.15*100=` to kick it off. Aug 19, 2017 at 15:10
• My HP19B needed something to prime the pump as well. Just found a video of a hack to the `=` button on a cheap calculator turning it into a counter: youtu.be/VLAv7cNCB3U Aug 19, 2017 at 15:15