But they change from time to time. For example, if you would like to know, how much seconds have passed since January 1, 1984 in a specific town / country, you need full database what was time zone each year, which years there was DST and which years not. In Latvia it would mean UTC+2 for some years, UTC+3 for others and also there was one year without DST. And also there was leap seconds in the past, at least it's decided to get rid of these.

And remember there is also 30 minute and 45 minute time zone offsets, not everybody uses +/- hours.