My question doesn't deviate away from the title (in fact, it is the title): Why are there two sets of the CTRL, Alt, and Shift modifier keys when only on one of each is necessary?
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When touch-typing, you use the alternate side "function" keys.
qwer,asdf,zxcv you'd use the right shift with your right hand without needing to stretch your left hand while pressing one of the aforementioned keys.
These modifier keys are very important for professional typists. If there was only one modifier key for CTRL, Alt, and Shift, then typing speed could be slowed down when one of these modifier keys was needed.
There is a concept of a "dividing line" which is supposed to be like this (and can be seen in the coloured keyboard image included below which comes from an instructional typing program which was probably designed for children):
(The same usage applies for the CTRL and Alt modifier keys.)
Unfortunately function keys are a bit of a mess depending on how the keyboard manufacturer designed the spacing, but traditionally they're grouped in three groups of 4 keys with F1, F2, F3 and F4 on the left-hand side, and F5 through F12 on the right-hand side, of the dividing line.