My Zojirushi BBCC-S15 works even though the window blinks 8:00. Why is a battery needed?

Some automatic breadmakers have a place for inserting batteries.

The intent is to create a memory back-up for two things:

1: The digitally-displayed clock setting, and
2: Personalized program settings you save to the machine to perform again later.

Now you're not required to install or replace the batteries in your breadmaker unless you desire these settings to be maintained long after the breadmaker is unplugged.

And, if you do decide to install or replace the batteries be certain to use alkaline-grade or better.

But be forewarned though: Breadmaker clocks don't adjust for DST (Daylight Saving Time)

Hope this helps!