its computer buying and selling- let say you want to sell at...

its computer buying and selling-

let say you want to sell at $1.45 but the bid is $1.39.

so you offer to sell 1 share at 1.44. you are pulling the bids up. better to get them to cross the floor for 5 cents than 6 cents. then the machine says last trade $1.44. new buyers coming along see $144 and $1.45 doesnt seem such a gap to cross

a very crude example and more reasons such as hiding larger orders in small trades and narrowing the spread closer to your curreny prices

they dont pay the $20 a trade we do.