i got one of similar kits (it was 10Hz-100khz function generator) but in the end it was qute buggy schematic + it had just one "cover-it-all" range so even with 10 turn pot. it was hard to get some low freq wave out of it.
My generator has an three range selector !!
About you last question ... " usability " Well , now that we have set all the options, of tools , on table ..
I will say that in my generator , even if it has three outputs on the PCB , my box has only one
The other two are connected with thin air ...
I work with repairs of sound equipment , mixers and amplifiers , for home use and large dancing halls .
Primarily what is needed are the acoustic range , 80Hz - 9kHz .
For acoustic frequency's testing , you need other the sound generator , one fasma-scope.
( well my English does not helping me here , but I am trying )
fasma-scope or fasmatoscope ... its a graphical interface , equal of what we see in some cheap, and high end EQ devices ( Equalizers ) , that they have one little screen in the center , with led bars , one for every "base" frequency ..
Kenwood had build one fantastic EQ , with 16+16 "base" frequency's and a huge "spectrum analyzer" display, on it .
With all this tools , I was able to check the total frequency response , of the device that I was testing.
So , the importance of the generator it self , has a little value , if there is no " fasma-scope or fasmatoscope or spectrum analyzer with display , at the end of the chain .
Ok ... theoretically , you can insert an single signal as 1kHz as example, and measure the output in mV ,with a simple multimeter , but this are the theory ... in praxis you need " real " tools = more advanced .
In SUM , you will need just one simple sound generator.