To power electronics within to run tone bleepers,displays,amplifiers and memories.
Batteries are used now more so that ever. All of the hand held controllers, telephones, game consoles, portable radios all need batteries for their source of electricity.
Because they run on cell batteries
Because you can use them multiple times.
Cellular Telephones is using the Integrated Services Digital Network ISDN while the Cordless Telephone is using the Public Switched Telephone Network PSTN, same with ordinary telephone.
Brass
brass
Yes there are cassette player with rechargeable batteries. Even if you buy cassette player that runs on ordinary batteries, you can just replace it with rechargeable ones.
Batteries be should not be disposed with ordinary trash due to their flammable nature. They should go with the hazardous waste for recycling.
For ordinary camera batteries, try the camera department of a big store such as Target or WalMart. For proprietary batteries (special batteries for specific cameras) try a camera shop or B&H PhotoVideo online.
Wireless telephones sometimes have interference due to low batteries or the base units being kept too close together. Contrary to popular belief, WiFi works on a different frequency and does not interfere.
carbon
no, it does not need batteries because it is powered by the sun.