Matthew 6:5

“And when you pray, you shall not be as the hypocrites,

For they love to stand and pray in the synagogues and on the corners of the streets,

That they may be seen of men.

Truly I say to you, They have received their reward.”

The disciples are warned against putting on an act in prayer. Among the Jews, to be seen as a praying man was a very desirable thing, because such a man was admired and respected by all. Thus those who wanted to be admired and to put on an act that they were pious stood up where they could clearly be seen in the synagogues, or on street corners (or public open spaces) at the time of prayer, and there made a great show of praying to God, even though they were only praying to themselves. Men and women then thought that they were wonderful. But God did not think that they were wonderful. He simply turned away in disgust. As far as He was concerned they had already received their ‘answer to prayer' by what men and women thought about them. They had had their reward. Compare here Luke 18:9).

It was not normal to pray on street corners as a general rule, but the point is probably that some arranged to be in such an openly observed position as a street corner when the hour of prayer came round, which was the time when all should pray, and would then stop and pray so that all might see their piety. For all would know that a truly pious man must observe the hour of prayer wherever he was. So his aim was that people would say, ‘How pious this man must be!' And so he had received his reward.

Note that it is his intention that is being judged here. It is not that he prays in public because something has prevented him from getting to the place of prayer. That could be commendable. It is because it was all the time his intention to pray in public, so that men would see it, and give glory to him instead of to God.