This was a merciful provision for the poor, in their times of offending. The Virgin Mary's offering upon another occasion, was in this humble way. Luke 2:24.
Related Commentaries of Leviticus 5:7
Leviticus 5:7
7 And if he be not able to bring a lamb, then he shall bring for his trespass, which he hath committed, two turtledoves, or two young pigeons, unto the LORD; one for a sin offering, and the other for a burnt offering.