Most of this is from possibly-faulty memory of things I may have never
actually learned in the first place, but:

Sun3's used actual EEPROM (aka EAPROM), probably this same SEEQ chip.
EEPROMs have a limited "write lifetime" (you can read from them forever,
but you only get ~10000 writes).

The Intersil chip is just a counter/timer.  Machines with a SEEQ and
Intersil thus have an actual non-volatile EEPROM.

The Mostek chips are actually battery-operated-wristwatch chips.  By
packaging the chip and a battery together, you get:

 - a BCD time-of-year clock (BCD because LCD watch displays use BCD
   decoders)
 - some battery-backed-up RAM

The latter serves as "EEPROM" and is infinitely writeable, unlike
true EE/EA PROMs.  It also loses its memory when the battery goes
dead.  If the battery is physically removable, this allows erasing
the chip contents, but the ones I have seen are impossible to get
at without destroying the Mostek chip.  (Kind of annoying, since the
battery only lasts ~10 years.)

Chris