Why does ShortPixel Adaptive Images tell me that my quota will be exhausted soon?

If you use ShortPixel Adaptive Images there's a chance that sooner or later you will see a notice like this one on your WordPress admin area:

Even though it might seem obvious why it's showing, if you purchased a big number of credits, you may get confused because it can start showing up while you still have plenty of credits. This is because the notice will appear when you have used more than 90% of your quota. So, for example, if you have a quota of 86,500 credits, when you will have less than 8,650 available credits you will start seeing the notice.