They are absolutely unnecessary. These serve only two purposes. It either benefits a site monetarily or there's some other nefarious reason for them. Merely clicking on the "X" or "SKIP" is an acknowledgment for whatever purpose the web creator designed them to do. Nearly 90% are cookie dependent. Meaning if someone simply refreshes the page they will go away. If they don't go away leave the site. None should be so important as might risk one's security.
Clicking on them provides a means for a web designer to acknowledge your approval for something you may or may not fully understand or agree with. Common sense should dictate why they would be necessary in the first place. In short, don't never ever do it
I've already ran across some that refused to let me close the browser. Here's how to get around this problem. Press the CTRL-ALT-DELETE keys simultaneously. This will bring up the "Task Manager". Chose "end the task" option for the browser. Restart the browser and use the tools menu. Under the internet options delete the files. Then close the browser, restart and repeat this procedure a second time and you'll be A-OK.
Above all else never ever acknowledge a pop up of any kind no matter what.