12,
Egg timers are used when you want to manually turn something on, and then have it automatically shut off after the egg timer times out.
There is no reason to use them unless you want to. Keep in mind that they count as a schedule, so if you have an ET lite PL4 or PSL4 then you only have 4 total schedules, and if you have a standard ET, you only have 12 total schedules.
I use one for my pool water fill line. I never know when I'll need water so it can't be scheduled to turn on, but I do know that when I manually turn it on, that I want it to automatically shut off after one hour.
If you are not going to use them, make sure they are set to their default of 12 hours.
Thanks,
Jim R.