They provide the basic functions of the site -- it wouldn't be possible without them, just like a house without a roof.

They help us show ads that make sense to you — no houses from the sea if you are looking for a cottage in the Highlands.

They allow sites to remember your choices — such as your language, name, or favorite city. Just like when we remember what you are looking for on a repeat visit.

Thanks to them, we know how you are doing on the site — what works, what to improve, and if something does not rub somewhere (like a door at an older house).