18 Paul stayed on in Corinth for some time. Then he left the brothers and sisters and sailed for Syria, accompanied by Priscilla and Aquila. Before he sailed, he had his hair cut off at Cenchreae because of a vow he had taken. Acts 18:18
Why is Paul’s haircut a detail in the Book of Acts? Because he took a vow.
Why are vows important? It seems obvious but vows are promises made. Presumably it was a vow to God.
Proverbs in the Old Testament and Jesus’ teaching in the New Testament tell us about the importance of keeping vows. More precisely those verses warn us about being careful to know what we’re vowing before making a vow (Proverbs 20:25, Ecclesiastes 5:4, Matthew 5:34-47). It’s certainly a sound practice in life.
The practical application to this is simple: be extra careful about making promises to the Lord.