Another cookie rant, but...
Cookies are used to remember 'things'. This is two fold and the second one I hate with a passion!!!
First the main purpose. They remember things like login IDs and shopping carts.
Imagine logging in to this forum them when you click on a post you want to read, the forum forgets you're logged in - how would you get anything done? Shopping carts; same thing - as you add items to the cart then proceed to their page to complete the transaction, it forgets what is in your cart.
Second, they allow tracking of their customers browsing, which pages they clicked and products they looked at. I dont mind people doing it to improve website services but there is potential for them to use it for targeted marketing and show you ads, or even send email, based on that click.
Also, they arent just website specific. The famous one is a cookie doubleclick (owned by google, but there are lots of others). Lets say I open a new online shop selling... garden sheds. Google pays me a small amount for me to have their doubleclick cookie track my visitors. So now google knows they're thinking about buying a shed and stores that little nugget of info on their computer. Now, facebook might use doubleclick too, so it will display ads on that persons News Feed about... you guessed it, garden sheds. How did it know ?!?!?!
The final pain-in-the-ass double-edged sword of cookies. Lets say you visit a site, the website says "oh, no cookie exists, lets show an ad pop-up" and then tries to save a cookie. If you accept the cookie, the pop-up might only show once every 5 visits (but then track your marketing preferences) but if you deny it, it will think you're a brand new visitor and show you the pop-up every single goddam time!!
I never ever use the B&Q or Homebase websites. They have lost an customer with me (unless I *know* they have what I want or I'm physically passing the store anyway). Their website has such a cookie policy that they INSIST on tracking what you view, so much that every page on their website redirects to an error page if you deny the cookie. Its just not required to browse a website, Im not going to buy online and have a shopping cart, nor am I going to log in, so they dont NEED to place a cookie. They just desperately want to track my browsing... to improve their site or sell my marketing footprint?? The world as it is, I dont like the idea of having a cyber-profile and having all my preferences and browsing tracked just so someone can give me what IT thinks is stuff im interested in (cos after I've bought my shed, I dont want to keep seeing ads for them!!).