Hello,
"What is the purpose of this relation? "
To depict that "Wall Street" is in reality represented in OSM with four (4) segments:
https://www.openstreetmap.org/relation/15806058#map=18/40.70601/-74.00901

"Seems like goes against..." 
Please note that Relations are widely used to bind scattered elements:
https://wiki.openstreetmap.org/wiki/Relation

"why did you add subway platforms to a street relation?"
That was I mistake and I immediately took action, it lasted for less then 5 minutes.
Subway platforms were accidentally added to a street relation and quickly removed:
https://www.openstreetmap.org/changeset/135689545

Note that I only add a cohesion element which does not interfere with anything else previously represented. 

Examples of the use of "type"="street" in NY area:
Please have a look in: https://overpass-turbo.eu/s/1uzF

Also please have a look on Westchester Avenue:  
https://www.openstreetmap.org/relation/14870829

Last and more important: 
If you feel bothered with this edition I have no problem on undo it. 

RB