NVRAM doesn't need batteries, that's why

Batteries wouldn't change how it behaves, it's stores data persistently without the need for power.

It's easier to think of it as equivalent to a flash drive that's plugged into your PC. You can put that in your pocket and the data is still on there, but that data never changes unless you manually change it (so a clock would never keep time).

Essentially NVRAM is a fit and forget thing, you never have to think about replacing batteries again.

That said I don't know how long Lithium batteries typically last in a pinball machine, I'd assume they would hold a charge for at least 2 if not 3 years.

How much is NVRAM and how much would it cost you to get it fitted (if you aren't capable yourself)? I assume that price would buy an awful lot of packs of Lithium batteries...

If money is no object then outside games that use the RTC it's a no-brainer.