Help me understand the penalty

I could use some help here, I am struggling to understand what happened on the 2nd and 26 Kamara screen pass play?

the run was 23 yards so it would have 3rd and 3, but there was a block in the back after the catch and the Rams accepted the penalty.

the Saints repeated 2nd down but now it was 2nd and 21. So they gained 5 yards and repeated 2nd down.

What I am having a hard time grasping is how a team could benefit from a penalty against them.