Dumb questions about GMT offset in tm structure

Dumb question #1:  what applications use the offset from GMT of local time?
(That is, why do we want the offset in the tm structure in the first place?)

Dumb question #2:  if there's a way to turn tm structures into time_t values,
does the ability to do something like
    offset = time_of_interest - convert_tm_to_gmt(localtime(time_of_interest));
(possibly followed by
    offset = -offset;
depending on what sex you think the offset should be)
obviate the need for a GMT offset element in the tm structure?
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