If this is going to be a pissing contest, I have been selling since 1961 and my first account was Coca Cola for a print publication. I've been a multi-station cluster owner and GSM going back to the 60's. A more recent station group I was, among other things, DoS and increased gross billings an average of 28% a year over a 10 year period.
Reach and frequency requires considerable math. I use either an old DOS version of the Westinghouse NuMath or the NuMath slide rule that Westinghouse sold to any station back in the 60's. I bought enough of them to give one to every media buyer and planner in my market back then... about 400 of them... and then gave a number of the local broadcasters' association seminars on buying r&f.
Reach & Frequency is a radio concept, too. And you can't have one without the other because you need to specify either of the two in order to get the other.
If a campaign's goal is the have a determined frequency (% of cume in demo reached the desired number of average times), then you have to have to look at both the cume and the TSL.
To begin with, there are light users of a station that are nearly impossible to reach successfully Efficiency drops considerably once you try to get a reach of over 65 to 70 as you need so many more spots to tet even a frequency of 1 for the occasional listeners. That's because in the diary world, on average, under 40% of the cume gives 90% or more of the TSL.
Most advertisers want to know the frequency as they also know that one weekly impression is not enough and there is a also a point where you reach the My Pillow 'point of overkill'.