It's completely random, many people get them everyday. It's jsut to show you waht features there are, and to make you at least consider, or buy, a deviantART subscription for longer.
Different things are:
You can have a "friends" list on your deviantART page;
You can see who has recently visited your page;
Advanced journal features, such as being able to put what your mood is, what song you're listening too, in your journals;
You can use HTML in your journals;
No Adverts;
Larger Thumbnails;
More thumbnails per page;
deviantART Mobile;
Faster Surfing;
Private Forums;
Beta Testing.
I know about all of these because someone bought me a 3 month subscription.
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