Cache and cookies are not the same thing
Cache is a temporary store of images and files kept locally so pages load faster on your next visit; cookies are small pieces of data that remember your login state and site preferences. Both take up browser storage, and letting either build up for a long time can eventually cause loading glitches or display errors, not just eat space.
Different from simply clearing your history
This is specifically about clearing cache and cookies, not your browsing history -- deleting history alone does not touch stored files or login sessions, and won't fix a broken or glitchy page the way clearing cache does. If you're trying to fix a slow, broken, or stuck-logged-in page across Chrome, Edge, or Safari specifically, cache and cookies are the two items to target, and knowing the side effects in advance keeps you from being surprised by a sudden sign-out.
Frequently Asked Questions
Does clearing cache also delete my saved passwords?
No -- unless you separately check a "Passwords and other sign-in data" option, clearing cache and cookies only removes those two items and leaves your saved passwords untouched.
Will clearing cache make my browser faster?
It can, if a bloated cache was slowing the browser down, but in normal use cache actually speeds up loading, so pages may feel slightly slower than usual right after you clear it until the cache rebuilds.