Lenses are one of the most popular features of Snapchat - after the ability to send videos/photos that expire after 10 seconds, of course. It seems that Snapchat has also noticed this, with the photo-sharing giants offering the ability for its customers to buy (with real life money) lenses to be used at any time. The firm has since scrapped the idea of selling Lenses, and to stop the feature becoming stagnated, has introduced a new system. Instead of offering the same range of lenses on a day-to-day basis, the selection is updated daily, removing the oldest Snapchat lens and replacing it with a brand new one.
Don't worry, though - if you did buy any lenses, these will still be permanently available to you.