Good question, good answers. Important thing to remember if you accept a Visual Approach Clearance from AppCon you are accepting the separation responsibility to follow the aircraft you are instructed to follow. The Controller may issue other instructions, stay 5 miles behind ( as others pointed out to make a departure slot for the Tower), issue safety advisories ie: wake turbulence, Traffic going to the parallel Rwy, speed instructions etc.
Things go faster and smoother because the situation is not as complex as when weather dictates Instrument Approaches are required.
The Herd is much easier to handle and is self separated "See Him, follow 'em, call the Twr". doing ILS's your vectored further out, more detail to speed and altitude management is required. This can be exacerbated by having to give the Twr larger departure slots as well. Instrument Approaches are simply more complicated and and require more effort on the part of ATC and Aircrew.