Part of it's defense against last-second 
snipers... it doesn't work very well otherwise. 
^_^ I'll go down to increments of .50 (if it's a 
small item), but that's about it. But, say, for 
instance, if I was a sniper trying to snag 
something at the last few seconds, I'd probably 
try and make a whole-number guess. $100. $125. 
$150. Something like that. Hence, someone trying 
to defend against a sniper would try and set 
their max bid at $112, $128, or $150.01, so that 
the sniper doesn't have time to go back and re-
bid. Of course, it's all pretty useless if the 
sniper overestimates your bid, or is just plain 
willing to pay more than another.

But that's my guess.

-Cres