I've had the same here for years. Our main landline phones connect to a base unit that needs mains power but I can plug this one in if we have a power cut - at least while phones lines carry power.

It's mostly as an emergency route for incoming calls these days as we 3 have our own PAYG mobiles. We give the home landline number to those who may wish to contact us. I know we differ from many on this these days but we don't want to be carrying mobiles around and having people (outside family) contacting us on our own mobile numbers.