James Farrar wrote:

The greatest advantage of the four-track system [1] is that it allows
24-hour running. The express trains often save you less time than you
might think.

Another advantage is capacity. A four-track line can carry (roughly)
twice as many trains per hour as a two-track line.

[1] Well, it mostly is... I was particularly intrigued by the
three-track layout on the 7 in Queens...

Many lines have three tracks, although only a few actually have regular
service scheduled to run on the middle track. On the others, the middle
track is still available for scheduled and unscheduled reroutes.
--
David of Broadway
New York, NY, USA