You can get several bins of leather from the first caravan that visits you, if you're experienced. You can also have reliable leather production from a livestock industry by the second year (sooner if you bring cows). You may get lucky and have a strange mood result in a legendary leatherworker, in which case you'll have suits of masterwork leather armor sitting around. It's available early and has a lot of little benefits:
1) Good lightweight armor is great for training dumb recruits, because it doesn't slow them down or tire them out, which means they get more training before they get exhausted.
2) Masterwork leather armor can approach artifact value in trades, depending on what kind of leather it's made from.
3) As leather approaches masterwork quality, the material/quality multipliers bump right up against zero-quality iron armor. I know the block value isn't as good, but it's still nice to get the full value.
4) Did I mention that the materials are much more widely available and cheaper? It takes 4 pieces of leather or 6 bars of metal to make a full suit of leather or chain, respectively. You can buy 2 bins of leather for a few hundred dwarfbucks (5 dwarves' of armor)... how many caravans will it take to get 30 bars of iron? ...steel?
None of this should take away from the fact that you eventually want your military dwarves in silk clothes, leather armor, steel chain mail, and steel plate (all of the highest quality). But early on, leather is a great way to more-than-double your average dorf's embark armor, or train him up