Prayer is for fruit-bearing. This is confirmed explicitly in verse 16. The logical connection between the two parts of this verse are tremendously important. Jesus says that he chose and appointed his disciples that they should go and bear fruit that remains . . . This only makes sense if prayer is for fruit-bearing.
Prayer is for fruit-bearing.
Therefore, since I want you to pray and to get answers to your prayers, I chose you and I appointed you to go and bear fruit. Because prayer is for fruit-bearing. If you are not devoted to fruit-bearing you have no warrant for expecting answers to prayer. Prayer is designed for fruit-bearing. Prayer is not for gratifying natural desires.