When you're drunk and just want something quick, hard to beat.
As someone who lives in the outer edge of an Outer Borough, my experience is somewhat different - most slice places close fairly early (10 pm on average in my area, slightly later on weekends) and long before bars do. As a consequence, my go-to when out and about is a chopped cheese from the local deli/bodega.
I've eaten at plenty of dollar slice places and honestly find them adequate for the price. I think I've only ever had one terrible slice, ever - and it wasn't a dollar slice place. I was unsurprised when the place closed shortly after.