Very often, praying in the spirit will cause you to pray in an unknown tongue. When you do this, you are allowing Jesus to pray through you for those things He sees coming that day. He also knows all the things that are in your heart, He knows everything that would hinder you from receiving mercy, and He knows what you need to know and be prepared for.
Hence when we pray in the Spirit, Jesus gets the chance to pray to God on our behalf, that He (God) will give us everything we need to walk in Him today. So if we need knowledge of what is stopping us from receiving mercy, Jesus will pray and ask God to give you that knowledge, and then the Holy Spirit will begin to reveal it.
When it is revealed all you need to do is obey. In other times, He may simply pray the will of God for that day. He ask God to give us the strength to combat what is on its way. God will always respond to His prayer. Hence it is always good to pray in the Spirit, because Jesus is speaking to God in your favor.