Albert Barnes Commentary Matthew 6:5

"And when ye pray, ye shall not be as the hypocrites: for they love to stand and pray in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have received their reward." — Matthew 6:5 (ASV)

And when you pray, etc. Hypocrites manifested the same spirit about prayer as alms-giving: it was done in public places.

The word synagogues, here, clearly means not the place of worship of that name, but places where many were accustomed to assemble—near the markets or courts, where they could be seen by many. Our Lord evidently could not mean to condemn prayers in the synagogues. It might be said that he condemned ostentatious prayer there, while they neglected secret prayer; but this does not appear to be his design.

The Jews were much in the habit of praying in public places. At certain times of the day they always offered their prayers. Wherever they were, they suspended their employment and paid their devotions. This is also practiced now everywhere by Muslims, and in many places by Roman Catholics.

It seems, also, that they sought publicity and regarded it as proof of great piety.