1. God commands it.
“And when you pray, do not be like the hypocrites, for they love to pray standing in the synagogues and on the street corners to be seen by men. I tell you the truth, they have received their reward in full.” Matthew 6:5

“This, then, is how you should pray: ” ‘Our Father in heaven, hallowed be your name, … Matthew 6:9

Then Jesus told his disciples a parable to show them that they should always pray and not give up. Luke 18:1

Devote yourselves to prayer, being watchful and thankful. Colossians 4:2

Be joyful always; pray continually; give thanks in all circumstances, for this is God’s will for you in Christ Jesus. 1 Thessalonians 5:17
2. Circumstances require it.
A. James killed by Herod and Peter arrested.

It was about this time that King Herod arrested some who belonged to the church, intending to persecute them. He had James, the brother of John, put to death with the sword. When he saw that this pleased the Jews, he proceeded to seize Peter also. This happened during the Feast of Unleavened Bread. After arresting him, he put him in prison, handing him over to be guarded by four squads of four soldiers each. Herod intended to bring him out for public trial after the Passover. So Peter was kept in prison, but the church was earnestly praying to God for him. Acts 12:1-5

B. The Apostle Paul imprisoned.

Now I want you to know, brothers, that what has happened to me has really served to advance the gospel. As a result, it has become clear throughout the whole palace guard and to everyone else that I am in chains for Christ. Philippians 1:12-13

…for I know that through your prayers and the help of the Spirit of Jesus Christ this will turn out for my deliverance. Philippians1:19
C. We are to pray “in everything.”
Do not be anxious about anything, but in everything, by prayer and petition, with thanksgiving, present your requests to God. And the peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus. Philippians 4:6-7
3. The times demand it.
And pray in the Spirit on all occasions with all kinds of prayers and requests. With this in mind, be alert and always keep on praying for all the saints. Ephesians 6:18

A. These are times of difficulty.

But mark this: There will be terrible times in the last days. 2 Timothy 3:1

B. These are times of deception.

The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons.1 Timothy 4:1
C. These are times of diligence in prayer.
Devote yourselves to prayer, being watchful and thankful. Colossians 4:2
4. Spiritual warfare necessitates it.
A. God’s kingdom is always vigorously opposed.

Therefore, since through God’s mercy we have this ministry, we do not lose heart. Rather, we have renounced secret and shameful ways; we do not use deception, nor do we distort the word of God. On the contrary, by setting forth the truth plainly we commend ourselves to every man’s conscience in the sight of God. And even if our gospel is veiled, it is veiled to those who are perishing. The god of this age has blinded the minds of unbelievers, so that they cannot see the light of the gospel of the glory of Christ, who is the image of God. For we do not preach ourselves, but Jesus Christ as Lord, and ourselves as your servants for Jesus’ sake. For God, who said, “Let light shine out of darkness,” made his light shine in our hearts to give us the light of the knowledge of the glory of God in the face of Christ. 2 Corinthians 4:1-6

B. Prayer is the primary way God’s kingdom advances.

“This, then, is how you should pray: ” ‘Our Father in heaven, hallowed be your name, your kingdom come, your will be done on earth as it is in heaven. Matthew 6:9-10
5. Destiny is determined by it.
A. God has given the earth to mankind.

The highest heavens belong to the LORD, but the earth he has given to man. Psalm 115:16

B. God changed Jabez’s destiny because he prayed.

Jabez was more honorable than his brothers. His mother had named him Jabez, saying, “I gave birth to him in pain.” Jabez cried out to the God of Israel, “Oh, that you would bless me and enlarge my territory! Let your hand be with me, and keep me from harm so that I will be free from pain.” And God granted his request. 1 Chronronicles 4:9-10
6. Nations can be preserved through it.
A. God promises to preserve His people and their land through prayer.

…if my people, who are called by my name, will humble themselves and pray and seek my face and turn from their wicked ways, then will I hear from heaven and will forgive their sin and will heal their land. 2 Chronicles 7:14

See Joel 2