If you were a leader of the confederate war effort, why would you direct southern armies to invade and capture a major union city, such as boston, new york, or philadelphia?

Possibly since they were allied with the union they may have been in an important role in making supplies (even tho they had a hard enough time getting it to them) if the confederates could take those towns they would cut off those major supplies from the unions. on a side note the union solders were more far more equiped with vital equipment other than just a springfield or enfield musket (which were standard for both armies) a cartrige pouch, a bag some ragtag uniforms and a bayonet ( unless the union had other places they can get supplies from they wouldnt stand a chance against the confederates...) but that was a risky decision from the confederate general couse it wouldve taken awhile since most had to walk and they would be exposed to the elements and diseases so some could die