Do commercial service airports use VFR approaches?

I fly United Airlines quite a lot, and the feature I like the best about their airline is you can listen to ATC conversations. Being computer flight simulator enthusiast I always enjoying listening to such conversations. A couple of days ago I flew on United 736 from KSFO to KIAD, and I noticed that the ATC... show more I fly United Airlines quite a lot, and the feature I like the best about their airline is you can listen to ATC conversations. Being computer flight simulator enthusiast I always enjoying listening to such conversations. A couple of days ago I flew on United 736 from KSFO to KIAD, and I noticed that the ATC assigned the flight to land on run way 1L using visual approach. My questions are:

1) KIAD is a pretty busy class B airport, how do the ATC determine either instrumental or visual approach is gonna be used? Is it solely based on the weather / clouds / visibility?

2) During a visual approach, do pilots take advantage of the ILS instruments i.e. follow the vertical guidance when they are entering the base leg and final turn?

Thanks in advance.
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