If your air gun is about the same as every other air gun I've seen, then pushing the nozzle against the hole should be enough to set off quite a storm through the fork legs.

If you try to braze/weld something that's a fairly small enclosed space, the excess heat from the process will warm up the air inside the fork leg. The warmer air will expand and try to blow out, usually through the gap that you're trying to braze/weld. makes it really hard to get a nice seam.
They might also be put in as drains.

In your particular case the lower holes might not have been entirely required, but odds are that the fork legs came like that regardless if the fork was going to be fitted with those higher mounting points or not.