That would be a question best posed to your teacher please.
The pose, which I believe is Paschimotanasana, has some requisite actions. It mandates that the student understand length in the anterior or front spine. It also mandates opening in the hamstrings. There is a different between a pose that provides hamstring opening and one that requires open hamstrings. This pose requires open hamstrings.
Typically the use of a block in front of the feet is for those practitioners who have correct actions AND can reach their feet with ease while maintaining those actions. Conversely, a bolster or folded blanket under the front of the torso is usually employed for students who do NOT have the requisite actions so that they can rest the body on the prop and find a bit of comfort in the pose. It is also used for restorative poses where the mission is to considerably reduce effort in asana.
Forcing the body to fold in this pose while keeping the legs straight can place undue strain on the hamstrings and the two places they attach in the body.
gordon