Summary: The bible exhorts us to “pray in the Spirit”. This can have a broad range of meanings. But there is one specifically encouraged biblical type of praying in the Spirit, and that is praying in tongues. Come read about the blessings that follow from this biblical practice.
Ephesians 6:18-19 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.  Pray also for me, that ... words may be given me...
•• So praying in the Spirit is not only praying in tongues, although, as we will shortly see, praying in tongues is one definite, specific aspect of it.
•• Praying in the Spirit can also simply be praying in a state of mind where you have shut out the cares and thoughts of the world and the flesh and have truly focused your soul and spirit on the Lord. One of many blessed benefits would be that we are "building up [ourselves] on [our] most holy faith, praying in the Holy Ghost" (Jude 1:20, KJV).
Romans 8:26-27 In the same way, the Spirit helps us in our weakness. We do not know what we ought to pray for, but the Spirit himself intercedes for us with groans that words cannot express.  ...the Spirit intercedes for the saints in accordance with God’s will.
•• If we are truly “in the Spirit”, then the Holy Spirit can bear our prayers before God under His inspiration, and according to His will — even when we can’t find the words to express our petitions.
•• A specifically biblical method of praying in the Spirit is praying in tongues. And to that Scripturally endorsed practice we now turn. [Note for clarification: the Scriptures reveal that speaking in tongues is the initial evidence that someone has received the biblical baptism with the Holy Spirit.]
1 Corinthians 14:2, KJV For he that speaketh in an unknown tongue speaketh not unto men, but unto God: for no man understandeth him; howbeit in the spirit he speaketh mysteries.
•• Speaking in tongues is speaking “in the spirit”.
•• 1 Corinthians 14:14 For if I pray in an unknown tongue, my spirit prayeth, but my understanding is unfruitful.
•• What benefits come from praying in the spirit, from praying in tongues?
1 Corinthians 14:2, KJV For he that speaketh in an unknown tongue ... in the spirit he speaketh mysteries.
•• Although “mysteries” to us, they are “in accordance with God’s will” (Romans 8:27). Praying in tongues, under the Holy Spirit’s inspiration, is prayer that is in tune with the will of God.
1 Corinthians 14:4, KJV He that speaketh [or prays] in an unknown tongue edifieth himself...
•• Praying in the spirit (that is, in tongues, in your prayer language) edifies you, even though you are not conscious of the prayer’s content.
1 Corinthians 14:14, KJV For if I pray in an unknown tongue, my spirit prayeth, but my understanding is unfruitful.
•• Praying in the spirit is an opportunity to set our typically dominant intellectual focus aside and pray to God from deep within, at the spirit-to-Spirit level. Our spirit is in intimate contact with the Holy Spirit, where the deepest communion between God and man occurs.
• Romans 8:16, ESV The [Holy] Spirit himself bears witness with our spirit...
1 Corinthians 14:15, KJV What is it then? I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also.
•• We can have a balanced prayer life, utilizing both our intellect and our spirit.
1 Corinthians 14:16-17, KJV Else when thou shalt bless with the spirit, how shall he that occupieth the room of the unlearned say Amen at thy giving of thanks, seeing he understandeth not what thou sayest?  For thou verily givest thanks well, but the other is not edified.
•• When you pray in tongues you may be blessing and thanking the Lord well.
•• In closing, permit me to comment briefly on a few verses in the book of Revelation. While not directly speaking of tongues, these Scriptures give us some further enlightenment on the general benefits of being “in the Spirit”.
Revelation 1:10 On the Lord’s Day I was in the Spirit, and I heard behind me a loud voice like a trumpet.
•• When “in the Spirit” you are better able to hear the voice of the Lord [Jesus, vs. 18].
Revelation 4:2 At once I was in the Spirit, and there before me was a throne in heaven with someone sitting on it.
•• “In the Spirit” you can get a clearer revelation of God.
Revelation 17:3 Then the angel carried me away in the Spirit into a desert. There I saw a woman sitting on a scarlet beast....
•• “In the Spirit” you are more sensitive to God’s revelation of evil people, false religion, and the work of Satan.
Revelation 21:10 And he carried me away in the Spirit to a mountain great and high, and showed me the Holy City, Jerusalem, coming down out of heaven from God.
•• When “in the Spirit” you can get a fuller revelation of God’s great future plans.