No one wants actual stock.  They want to gain money on price differences in stock, or get the dividends that owning stock gives rights to, or I suppose they want to be able to have the voting rights stocks grant.<p>The difference is all about timing.  I may want something else more than you do but am willing to sell now.  If at the time you close out your trade (that is sell the shares from me) the price may have risen or fallen.  If it rose you won and I lost by not holding longer.<p>This time mitigation is precisely what market makers have always done and what HFT market makers have driven the profits (and thus the costs to outside participants) out of.