In utkatasana, the quadriceps are strengthened (this is one of the most under-utilised muscles!). The back muscles are also strengthened. The focus is on dropping the tailbone to the earth and keeping the back long.
In this modified bhujangasana, the focus is on strengthening the back muscles, in particular, the erector spinae. The hands on the gluteus focus the attention on engaging the legs and lifting of the torso with the back muscles rather than pushing up with the hands.
The strong back muscles are for balancing out the strengthening of the rectus muscles of the front ('six pack'!).