Many computer programs need to deal with historical dates. Without these tz rules captured into one place, it would be very difficult to determine when an event happened in another timezone. Also, most data is either going to be stored in UTC or in some domain-specific alternate "center" (New York time, for instance), forcing conversions if a user wants to view the data in their local time.
A contrived example: Say I'm in London and I want to know what time the NYSE opened up in London-local time on Black Thursday, October 24, 1929. That requires looking up the opening time in NYC and then converting the time to a different timezone, possibly taking into account DST in both locales.
A contrived example: Say I'm in London and I want to know what time the NYSE opened up in London-local time on Black Thursday, October 24, 1929. That requires looking up the opening time in NYC and then converting the time to a different timezone, possibly taking into account DST in both locales.