Usually, people clear their histories to stop autocomplete from suggesting websites that they no longer use, or to keep their activities private from other people who use the same device. Of course, over time, saved data (including your history, cookies, and cache) builds up, taking up much-needed space on your hard drive and potentially even having an impact on your browser’s performance.
Additionally, some websites send data from one page to another using their URLs. For example, if you search Google for “clear browser history”, the URL that’s recorded will include “&q=clear+browser+history”. This makes it easy for people to tell what you do on specific sites. Further, theoretically anyone with access to your device could use your history (in conjunction with saved cookies) to access specific pages of a site you use that would normally require a login.