The only thing Hannah wanted was a child. Can you imagine not having the one thing you want? She was taunted by others for being barren which constantly broke her spirit. She decided bargaining with the Lord was her best bet. If the Lord would only grant her a son, she would give him back to the Lord and he would dedicate his life to the Lord. The Lord grants her wish and she honors her promise. Simple as that.
 
We sometimes ask for things or want things God may never bless us with. But when our hearts are in the right place, when our asks will ultimately give Him glory, there's certainly opportunity for Him to grant our wishes. Not to say He won't bless us otherwise, but if our asks are always about us and never about Him, then what's the point?
 
Scripture reference for Hannah: 1 Samuel 1