Does my flagstone Need to be re-sealed?

by Eve
(Los Angeles)

Hi,

I have a contractor putting in an extension on our flagstone patio, and he will be sealing the whole thing to give it a more unified look. At the same time he is suggesting that I have all the other stone on our property re-sealed, including the front walk and pillars leading to the entry door.
Can you please tell me if this is a cosmetic thing or a real NEED, to protect the stonework? It seems like stone should be able withstand anythiing (it is stone, afterall!), and we live in Southern California, so its not going to be subjected to freezing temps or ice. It is, however getting direct sunlight every day of the year.

Please advise! I have no idea what to do, and don't want to get sold an extra service if its not really necessary.

thank you!