What do execution guys do in a quant hedge fund's pipeline?

[powerforward]

Seems to me that quant research (statistical analysis and playing around with data) and engineers (maintain the libraries and other software that can help the fund make $$$) are the big moneymakers in a quant hedgefund



Outside HFT, what in the pipeline do execution guys contribute to? What does a day to day look like? What is their interaction with quant research? Do they sit in Quant vs mid-office w/ ops?