The following are quotes from some articles written about this question.. . Why tithe?  
Please see the links, and read the articles in full.

1.  ” . . . A New Testament teaching on giving which may be helpful to you is found in 1 Corinthians 16:2: “On the first day of every week, each one of you should set aside a sum of money in keeping with his income.” This passage brings out four points: we should give individually, regularly, methodically, and proportionately. . . . “   DOES A CHRISTIAN HAVE TO TITHE?  – Billy Graham Evangelistic Ass.   2.  ” . . .One of the mysteries in my wealth management practice is the glaring coincidence that a large majority of my wealthiest clients are some of the largest givers and tithers I have ever met. . . . Avoid getting caught in the web of excuses I find ensnaring many families  My favorite excuse is, “I’ll tithe when I finally make more money, or when I receive that promotion or raise.”  Can I be frank with you? No, you won’t! . . . . If you haven’t been able to give $400 out of the $4,000 you make every month, what makes you think anything will change if you make $6,000? More money doesn’t fix anythingl  Don’t take my word for it.  Take it from one of the wealthiest men in our country’s history, John D. Rockefeller, who said, “I never would have been able to tithe the first million dollars I ever made if I had not tithed my first salary, which was $1.50 per week.” . . . . Share these fundamentals with your kids and teach them to set up three labeled piggy banks: Church/Charity, Savings, and Everything Else.  In the first piggy bank, place 10% for the charity and/or church.  In the second, put 20% to savings, and put the rest in the last piggy bank.  This will be a tangible reminder that you first allocate money to the most important things.  They will grow accustomed to living on 70% instead of living on 105%, like many families do today. . .”    WHY I TITHE . . . AND SO SHOULD YOU – Andrew McNair, Founder and CEO of SWAN Capital

3.  “. . . Tithing is one way to worship God—to honor Him as your provider and remind yourself that all of your resources belong to Him and are provided through His grace. As you give away that first 10% of your income, you set your course to honor God in the way you handle the rest of your finances. . . .”  WHY IS TITHING IMPORTANT? – IBLP