In Robert Morris book “The Blessed Life” Robert says:

“Biblical giving always requires faith to give to God first.  That’s why so few Christians experience the blessings of tithing.   It means giving to God before you see if you are going to have enough.  By tithing, it is as if we are saying to God, ‘I recognize You first.  I am putting You first in my life, and I trust You to take care of the rest of the things in my life.’  That is why tithing is so important.  It is the primary way we acknowledge that God is first.”

So when we deliberately and intentionally give of the first of our income, instead of giving from what is leftover, it helps to develop our faith.

Luke 6:38 (NLT)

"Give, and you will receive. Your gift will return to you in full—pressed down, shaken together to make room for more, running over, and poured into your lap. The amount you give will determine the amount you get back.”