One of the many functions of the word ὁ is to turn a phrase into a noun, and one of the of the most common phrases is the prepositional phrase. Matt 24:17 show several examples of this, and it also provides an interesting exegetical issue. If this verse is about Jesus' return and the end of time, then why are the people told to not go back into their houses? Time is over; it doesn't matter what's in the house.
How an article makes a prepositional phrase into a noun
Bill Mounce