She needed to pick up a prescription from the drug store; a flashlight, batteries, and tape measure from the hardware store; and milk from the grocery store.

It is also used when you have a sentence with more than one subject, almost like two sentenes you're pushing together such as:

She was always the one to do the shopping; her sister was utterly useless.

Am I even hitting close on this one? How about htis setence, one I just wrote...which is it a comma or semicolon?

Kay did not trust her sisters, she never had.

I'm leaning toward comma but I'm having doubts.

Dorca pressed the cloth against the wound on her leg. She was bleeding like crazy.

Dorca pressed the cloth against the wound on her leg; she was bleeding like crazy.

Dorca pressed the cloth against the wound on her leg, and she was bleeding like crazy.

All of the above are grammatically correct. Not necessarily good writing, but grammatically correct.