First of all, I invite you to read Book IV of the Catechism of the Catholic Church, which is devoted to prayer, especially the first part of Book IV, which addresses your question thoroughly. You can find the entire catechism from vatican.va.
There are many reasons we should pray, and pray regularly. I would just point out two:
Firstly, Jesus himself prayed a lot. In the gospel accounts, especially Luke, Jesus was often said to go away to a lonely place to pray, very often for the whole night. He prayed during his 40 days of fasting; he prayed all night after healing many in Capernaum; prayed all night before choosing the Twelve; prayed extensively in the garden of Gethsemane before his arrest.
This is not easy to understand. If Jesus is God, and the Father and the Son and the Holy Spirit is one Trinity, why did Jesus need to pray? Weren’t the Three Persons always in perfect communion with one mind and one will?
I suspect that it was for two reasons that Jesus prayed: one is for his own humanity, another is to set an example for us. If Jesus the Son of God needed to pray, I definitely need to pray.
His humanity needs to pray, this is really saying that our humanity needs to pray. Jesus’ humanity is our humanity. In order for his humanity to be completely conformed to the Will of the Father, it needs to pray. It needs to be choosing intentionally to be in the presence of the Father. This reveals the principal reason for prayer: to know and conform to God’s Will. Our flesh is rebellious, it does not automatically conform to our will. It needs exercises to get into shape. We already know that in the physical sense. It is also true spiritually. Our human nature cannot be aligned to God’s design unless we regularly and diligently subject it to prayer.
There is a lot more I can say on this. But suffice it to say right now,is that prayer is the necessary antidote against spiritual sloth and rebellion, against the spiritual gravity toward self-will and sin.
The second reason for prayer is that Jesus taught extensively about prayer. In Luke 11, Jesus told the parable of the friend visiting at night after bedtime asking for bread, that even though the person might not get up from bed to help him because he was a friend, he would get up because of his persistence. Then He went on teaching us, “Ask and you will receive. seek and you will find. Knock and the door will open to you… If you who are evil know how to give good things to your children, how much more will the Father give the Holy Spirit to those who ask for it!” This passage is worth reading and rereading. The same teaching was given again with a different parable on the widow and the unjust judge in Luke 18. And let’s also not forget that Jesus taught us a prayer to the Father, which today we call “The Lord’s Prayer”. It is clear that it is vitally important for Jesus’ disciples to pray, and not just pray, but pray incessantly, persistently, and faithfully.
And in case we misread, the above teachings were not about asking for things mainly, but asking for good things, and always have seeking the Will of the Father in mind.
Your second question is on the effectiveness of prayer. It is a common experience that people pray and seem to receive no answer. People as a result lose faith because they do not receive the things they want.
While Jesus did encourage us to pray for what we need, Christian prayer in essence is not about things, but about relationship.
A good analogy is how children relate to their parents. Children often come to their parents for things, but good parents would not always grant them. Their relationship is not about things, but about love and responsibility. Good parents would try to teach and redirect their children toward desiring good and healthy things, and avoid dangerous and harmful things. They would deny them unhealthy food so they can have appetite left for the healthy meal. They would delay certain entertainment or snacks in order to train them to be responsible and sensible.
Prayer is the same for us with God. While as beginners, we may ask for a lot of things we want, but ultimately, God wants us to slowly learn to seek what He wants. Prayer is therefore not just asking for a list of things, albeit some really good and selfless things, but about wanting to be with God, to trust that He loves us, to know God’s desire and plan, to consult with God, to conform to God. Prayer is trust in action. We come to Him to entrust to Him our lives, so He may bless us with His own divine life.