In אז ישיר we say - נחית בחסדך עם זו גאלת. You led with your chesed, this nation you redeemed. Whose chesed? The chesed of the Jews. Because Hashem saw that we were baalei chesed, He redeemed us. [See Chofetz Chaim Al Hatorah page 108].
That would explain beautifully why we say at the say הא לחמא עניא and invite anyone to our seder. כל דכפין ייתי ויכול! Since we were redeemed because we are people of chesed it behooves us to invite poor guests.
