The 99% reward advances the Progress Bar from its current position to the 99% mark and just a handful of spins away from levelling up and a prize on Wheel of Spinz.
To get the most out of the 99% reward, use it wisely. For example, if your Progress Bar is currently at 80%, you only gain 19% if you claim the 99% reward at this point. In this case, it may be better to play through your current level and then claim the 99% reward as soon as your Progress Bar resets to 0%. This way, you can gain the full 99% of this reward.
But don’t wait too long. All rewards have a 24-hour expiration date so better to take some value from the 99% reward than none at all.