The reason to use tab was file size for one
This is urban legend. The percentage of 512-byte blocks that
tabs would save was never significant.
(I agree that tabs and--especially--newlines can significantly
compress fixed-field formats from punched-card tradition, but
on the tiny Unix systems where tab conventions were
established, big tabular files were very rare.)
Tabs were a convenience for typists. Of course the tty driver
could have replaced them with spaces, but that would have
foreclosed important usage such as tab-separated fields and
run-time-adjustable tab stops tab-separated fields.
(I have run into latter-day trouble with selecting a space-substituted tab
from a screen, only to discover that I was copying or searching for spaces
instead of the tab.. That's not an intrinsic problem, though. Editors like sam
handle it without fuss.)
Doug