Why? It fixes a multitude of problems.
Today for example I went to write a new post like this one and the page for a new post was completely blank other than a symbol showing the spell-checking was working.
Clearly this was a bug with WordPress being updated yesterday, so I deleted my browsers cache and cookies and tried again and the page worked perfectly.
Last week another issue happened with the same computer/same browser and it kept scrolling after I had scrolled a little bit with my Logitech mouse. I tried to get the page to stop scrolling by closing and reopening the browser but it didn’t work. So I deleted the cache and cookies and it worked again.
The first thing you should do with a browser problem is to clear the cache and cookies. Then disable extensions, and then try another browser. You can also try opening up an incognito/in-private window as well. That helps more with issues with authenticating to web-pages. Then if none of these work, you can uninstall and reinstall the browser although that is less of an issue the last few years. This was more common with IE and older versions of Chrome.
If you browse all day like I do to gather information for clients, you may want to delete your cache and cookies on a regular basis like each month to prevent these kind of issues. You can also set the browser to delete your cache/cookies when you exit with some browsers so that might save you time as well.
Web browsers still have things that go wrong, and now you are better equipped to deal with these issues.