I don't know of many people who love to stand and pray in front of everyone. There are certain types who are addicted to the praise and attention of being in the public ministry--and the Pharisees would definitely fit that bill--but I think most people hate the stiff awkwardness of having to pray out loud before others. So it helps that Jesus instructs us not to be as the hypocrites who do these things to be seen by men. He's warning against anything that smacks of that sort of hypocrisy, when you pray to impress anyone else but God.
I don't know about you, but my biggest problem in prayer is that I feel like I'm trying to impress myself. I dislike praying in front of others (but I'll do it anyhow if I have to), yet even in private I wonder if this very dutiful prayer time is really about speaking to God or if it's just about feeling good for having prayed. I guess that's why I feel a little tripped up by too much instruction on how and what to pray. I've heard of using the acronym CAST (confession, adoration, supplication, thanksgiving) to guide your prayer time. Then others come along and argue that ACTS (adoration, confession, thanksgiving, supplication) is the more appropriate order. But admittedly for me, when the time becomes too structured I start focusing on accomplishing the task, and then I get into "impress myself" mode and lose sight of the fact that I'm supposed to be talking to God. Prayer sure is a psychologically delicate thing.
That's why I really like Jesus' description of how prayer should be done. He makes it sound like such a close, intimate time between you and God. You go into a private room and close the door to make sure no one will see you. If you remove the possibility of being seen in the first place, there will be less struggle over whether you're trying to impress anyone. Then you pray to your Father "who is in secret" and your Father "who sees in secret" will repay you. The way Jesus describes it, it's almost like there's some conspiratorial delight God gets out of this secret meeting with you. Just the fact that he's called the "Father who is in secret" implies that a big part of his relationship with you, maybe even the best part, is what takes place outside the knowledge of anyone else but the two of you. He likes intimacy. He likes secrecy. He's kind of like that friend who's always cracking inside jokes to remind you that there are some things between the two of you that no one else will ever be a part of.
I also see subtle instruction here about how to think of God when we pray. Pretty soon we'll be looking at the Lord's Prayer that addresses "our Father who is in heaven." But in this text Jesus says your Father is also close to you, crammed next to you in your prayer closet as the Father who is in secret and sees in secret. In fact, he's even closer than that since he dwells inside you through the presence of his Holy Spirit. So when you pray, it helps to know that even though he is the Father of heaven, he is also nearer than your own breath. No need to shout up to the heavens and plead to be heard by the God who floats among the celestial bodies. He's close, very close. Before you say a word he knows it. Before you think a thought he's discerned it. Some of those words and thoughts he may have even planted there himself. And once you stop worrying about whether you're being heard or understood or noticed by God, you can relax and focus on speaking to him from the heart.