20% are a great way to a) pursue your own passion b) try out a new product area c) try out a new type of job
Note that 20% is not a reason to do "nothing". Based on your goals, most people use 20% to build upto something. (a) can lead to a new product, (b) can lead you to gain knowledge of a new domain (c) can help set you up for a different role if you want to switch.
20% are a great way to a) pursue your own passion b) try out a new product area c) try out a new type of job
Note that 20% is not a reason to do "nothing". Based on your goals, most people use 20% to build upto something. (a) can lead to a new product, (b) can lead you to gain knowledge of a new domain (c) can help set you up for a different role if you want to switch.