Offering is part of our worship to God, knowing that He is the Lord that we serve. Our offering to the Lord is something that we give back to the Lord, so that He may continue to use it according to His perfect and good will. 
2 Corinthians 9:6-8 reads, "The point is this: whoever sows sparingly will also reap sparingly, and whoever sows bountifully will also reap bountifully. Each one must give as he has decided in his heart, not reluctantly or under compulsion, for God loves a cheerful giver. And God is able to make all grace abound to you, so that having all sufficiency in all things at all times, you may abound in every good work."