The reason is, say you earn $400 at a per diem job. $400 x 26 = $10,400 annually, so they will withhold taxes for that tax bracket which is only 10%. But in total, if you have a $120k primary job and add this $10,400 on top of that, you are well into the 24% tax bracket, and your average or effective tax rate might be around 16-17%. That's why I put 0 allowances + $50 at the per diem job to increase the taxes withheld closer to the 16-17% so that I won't owe any more taxes on this per diem income on my tax return.