Personally I got a true worthy function generator in
May 2012, as expansion regarding professional grade testing capabilities at ITTSB.
But the truth is that I have built my first
basic frequency generator, due a electronics KIT, when I was just a kid in the age of 17 years old.
For audio and RF/FM band gear testing.
This link includes some good but very basic info, for understanding what is a function generator, what can it be used for, and how it works.
https://www.radio-electronics.com/info/t_and_m/generators/function-generator.php About my own analogue monster you can see some pictures here.
https://www.ittsb.eu/DMM-Test-tools.html