Why do I have three phase electricity?
Here's an example:

My home was provided with a three-phase supply when it was built in 1990 because the builder intended to install an electrical storage-heating system. 

I bought the house before the heating system was installed and opted for an oil-fired system instead. 

So, in my area, it's an option you can request from the power company if you have a specific need for it. 

So far as I know, three-phase supply is commonly used for equipment that requires substantially more power than most domestic household appliances. Some industrial electric motors require a three phase supply. In my area a single phase is 230 V so domestic households almost always use only a single phase. I expect the phases are separated at the local substation where high voltage local distribution is reduced to 230 V.