Answer:
It could be used for products. People could say, "These are 30% off", "They (any product) have been increased by 15%"
In psychology (i am currently doing psychology), they usually like the results in percentages, especially in the discussion part, where you discuss the results. You would say that (this is an example from my recent report):
"The participants that utilised divided attention got an average of 1.6 answers correct, compared to participants who utilised selective attention, who got on average, 4.7 answers correct. This is a 63% decrease...."