Matthew 6:3 But when thou doest alms, let not thy left hand know what thy right hand doeth:
4 That thine alms may be in secret: and thy Father which seeth in secret himself shall reward thee openly.
Why would Jesus give such a commandment?
I believe there are two reasons:
First, it saves us from the wrong motivation of giving for public applaud, rather than out of love for the brethren.
This wrong motivation not only clouds the act of giving, but persists beyond that, as the person who is known to have given lavishly goes around as if he has a special halo around his head!
Second, it saves us (specially church and ministry leaders) from valuing others according to the size of their giving.
It is so easy to give special honor to the rich because they can give larger amounts than their poorer brethren.