Basic negotiable securities tradings actions are considered elsewhere.
Role in markets
Intermediary traders buy and sell items, holding them for a short time while it changes hands between long-term investors. In doing so, they make a profit.
So, they focus strongly on the guessing and taking advantage of the motives of investors - aka front-running investors.
Liquidity, cost
The benefit they provide is in bringing greater liquidity to assets, buy supplying a sea of ready purchasing and selling power. So, an investor wanting to find a seller or a buyer has a easier time.
The disadvantage is that they increase the costs to investors: sellers receive less while buyers would pay more than if they were dealing directly with each other.