One of the cache’s that Sitecore has is the Media Cache. Whenever you use an image from Sitecore’s media library, Sitecore will retrieve the image from the database, scale it to the size you requested and then store it to disk in the media cache folder. On any subsequent request the image will now be retrieved from the media cache rather than the database.
By default each content management and content delivery server will locate the media cache in /App_Data/MediaCache
This is a relatively logical place to store a cache for images that wont cause you many issues. However, if you have automated deployments setup then you are likely wiping the whole of your website folder each time you do a deploy to ensure that your deploy remains the same on all environments. As the App_Data folder is in the website, your Media Cache will be deleted too.
Depending on your site then deleting the media cache potentially isn’t much of an issue. After all it’s a cache so all that will happen is the images will get cached the next time they are requested. But depending how many images get retrieved at the same time this could slow down performance, particularly if your content editors decided to put every image they ever uploaded into the same folder. Opening the that folder in the admin will create a nice amount of load on your server, particularly if you also have some extra image optimization logic installed.
Like most things in Sitecore though, you can change the location through a config setting.
In Sitecore.config you will find a property called Media.CacheFolder. Change this to somewhere outside of your website folder and Sitecore will now start storing the Media Cache in this location and it will be safe your all your deploys.
<!-- MEDIA - CACHE FOLDER The folder under which media files are cached by the system. Default value: /App_Data/MediaCache --> <setting name="Media.CacheFolder" value="/App_Data/MediaCache"/>