I quoted the top of the ASCII table to emphasize that these
numbers characters were mapped to were not at all abstract,
they were intended to be sent down a teletype wire (and the
reason ASCII left the 8-bit unused was so it could be used
for parity checking).
Also, the NUL charcter plays an important role in our story, because it was used as a string terminator in the C/Unix world. More on that later.
Anyway: uppercase, lowercase, some punctuation... what else more could you need?