The feat would expand the range to a 19-20 roll for a critical threat. As the feat suggests, it adds an increase to the threat range for a given weapon equal to the range it had previously. So, in the case of the bow, it has a threat range of (1) or 5%. Add another 5% and you get a range of (2), or a 19-20 on the die. And having a 10% chance of to crit for a successful attack roll is much better than 5%, especially since the bow has a x3 crit multiplier.
EDIT: Chongo beat me to it, and of course his answer is correct and far less wordy than mine.