Why are poinsettias pinched?

To control appearance and to improve the quality of cuttings are the two (2) main reasons why poinsettias (Euphorbia pulcherrima) are pinched.

Specifically, poinsettias can develop long stems and sparse foliage if such environmental conditions as exposure to heat, light and moisture are not monitored to their liking. Pinching an inch/2.54 centimeters off the stem tips about every four (4) weeks until the end of August/middle of September encourages the plant to produce more stem branchings and more flowers. Additionally, it improves the quality of vegetative cuttings because it increases the number of places from which cuttings can be taken and it makes for thicker, stronger, healthier stems.