All those little pictures you see when you go through an add-on get stored to save time to re-display, so if you are running slow it may be a good idea to purge those thumbnails from time to time. Thumbnails do add up over time, taking up valuable storage. Also the packages are what Kodi installs from when an add-on updates. Sometimes you might want to keep packages around if an add-on updates and doesn't work, so you could roll back to a different older version. Same thing there, you can recover storage by purging packages.
For the most part, purging thumbnails and packages and clearing cache won't hurt.