<p>Institutions, Hedge Funds, Banks, Corproations, etc all need to buy and sell securities for a variety of purposes, including hedging. They come to S&T desks and ask for two-way prices, ie in FX Options, a custy may come to me and ask me GBPUSD 1w ATM in 80mio GBP, I quote 8.5/9.5 (% vol), and the custy decides to give me (I.e. sell at 8.5), pay me (I.e. buy at 9.5) or pass my rate (maybe he got a better bid or ask from another bank). </p>
<p>S&T desks at banks are market makers, providing liquidity.</p>