There are a handful of situations when clearing Safari’s cookies or cache–one of the places where Safari stores website data to avoid having to download it afresh each time you access a site–can help improve your browsing experience.
The first is when websites simply stop working when you access them on Safari. Perhaps Facebook stops updating with new posts, for example, or the images on a site don’t appear as they should. If this happens, it’s likely the cache has become corrupted.
Alternatively, if you find that personal info automatically completed on sites isn’t correct, deleting cookies can help. These are small files that sites use to save data about you and what you do on the site.
Read more at Macworld.com