let's say you want to test and/or adjust an amplifier or filter.
you could bring signal from function generator to input, connect some sort of device to measure output (oscilloscope, rf probe or whatever) on the output and test the stage. now you could do this by hand (set one frequency and measure output, set another slightly higher frequency and measure etc.) until you are finished. then you make small adjustment and repeat the whole thing from beginning. as you can see this could take a long time. so wouldn't it be nice to have some sort of automatic system that changes frequency over and over in range?