The Mitzva of giving Tzedaka is learned from פרשת ראה. It comes from the words פתוח תפתח, that we shall surely open our hands to the poor.
The של״ה הקודש says that if we did not help the poor, they would have no choice but to resort to stealing. The idea of forgiving debts in the Sabbatical year is also a form of charity and is also mentioned in the Parsha. It is also derived from the wording of the Torah that really everything belongs to Hashem, and by giving charity we are giving back what is His.
The Rabbis say that צדקה תציל ממות, that charity saves from death. Rav Elazar Abuchatzeira זצ״ל used to say that if Tzedaka saves a person from death, it also saves him from the IRS and from jail, and certainly from sickness.
Hashem wants to see generosity on our part. The number of times that we open our hands to the poor, so will Hashem open His hand to us. We must never minimize the importance of this Mitzva. Shabbat Shalom