What does the annual fee for the “premium” version cover?

This is more of a sales question… sorry, I couldn’t find anywhere else to get an answer to this question:

So far I like the free version of this plugin and am considering upgrading. I’d like to know exactly what the $39/yr. fee covers. Does it mainly cover maintenance and upgrades, or is it a software subscription? Will my software cease to function at the end of the year if I don’t renew?

The page I need help with: [log in to see the link]