And to pursue Jarvo's thought:
You could argue you don't need a D7 anywhere except in 1st position (guitar), but there's lots of cases where shifting a C7 up 2 frets is a "better" option.
It's all about choice.
Two French sayings:
Il faut souffrir pour etre belle = One must suffer to be beautiful -- No.
And
Qui peut le plus peut le moins = If you can do hard you can do easy.
On that basis you'd do the Tour de France before cycling to work.
I prefer Horses for courses.
You could argue you don't need a D7 anywhere except in 1st position (guitar), but there's lots of cases where shifting a C7 up 2 frets is a "better" option.
It's all about choice.
Two French sayings:
Il faut souffrir pour etre belle = One must suffer to be beautiful -- No.
And
Qui peut le plus peut le moins = If you can do hard you can do easy.
On that basis you'd do the Tour de France before cycling to work.
I prefer Horses for courses.