Batteries for Phone

[#1]

I bought a landline phone nearly two years ago and I noticed it was flashing and warning me that the batteries were low I located them at the back of my phone and changed them,I know it may seem silly question but what are the need for batteries on a phone.

[#2]

Powering memories and displays etc

[#3]

Some have a hands free facility or backup for answerphone messages.
Some cordless base stations have the option for a battery backup so they carry on working in case of a powercut.