If when the Scripture mentions "praying in the Spirit" it means praying in tongues, ie: Paul says in 1 Corinthians 14:15, "I pray
with the Spirit (tongues) and I pray with the understanding (his natural language), then if a person never prays in tongues, all he can do is pray with his natural understanding, and never prays in the Spirit, so that he is never sure whether he is praying in the will of God.
Jude 1:20 says,
"Building ourselves up in our most holy faith, praying in the Holy Spirit". If a person prays just with the understanding, he might not be building himself up as effectively as he would if he prayed in tongues.
Look at Ephesians 6:18:
"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 Lord’s people.' So, praying in tongues is part of the armour of God!
1 Corinthians 14:14
For if I pray in a tongue,
my spirit prays but my mind is unfruitful.
1 Corinthians 14:2
For one who speaks in a tongue speaks not to men but to God; for no one understands him, but he utters mysteries
in the Spirit.
Philippians 1:19
For I know that through your prayers and
the help of the Spirit of Jesus Christ this will turn out for my deliverance,
Zephaniah 3:9: "For then I will restore to the peoples
a pure language, That they all may call on the name of the LORD, To serve Him with one accord."