Annie, google summary (as in the report Samwan has posted) shows that
"Recent research, particularly from Giving Evidence, strongly suggests that royal patronage does not significantly increase a charity's income, with studies finding no material difference in revenue for charities losing royal patrons, and concluding that seeking them for financial gain is often a waste of resources, though other benefits like reputation and morale can exist"
I should also say, I feel very strongly that the royals can pick and choose charities they consider worthwhile - it's hardly fair to charities as a whole, is it?
This aspect imo really should be a thing of the past, although you will see upthread I am not a Republican, but want a more modern monarchy.