Nehemiah and the Priests establish a covenant with God after their restoration to the land because they wanted to rather than out of obligation or coercion. This is the same attitude we should bring to the promises we make to God in our own relationship with Him.
God takes the promises we make to Him as seriously as the promises He's made to us.
Why we are doing something is as important as what we are doing.
Giving is a tangible demonstration of where our priorities are at in supporting the works of God we benefit from.