The human organism can be affected by stimuli, plants, drugs, medicines, vibrations, etc. in a multitude of ways. The rectal tissue is rich with nerve endings and is capable of absorbing a wide variety of compounds. These absorbed compounds will exert "whole-body/systemic" affects which may be different from edibles, tinctures, inhaled cannabis. The feeling will be centered around this area and radiate outward. The "why" is rather complicated and I will submit a better "why" answer after 1) doing more research 2) trying a rectal suppository myself.
Another reason there is a difference in what you feel when using a product that is inserted rectally, is that the drainage of the veins, where the product is absorbed, differs from other routes.
There are three veins that drain the rectal area. The superior, middle and inferior rectal veins. The superior vein drains into larger veins that lead into the liver. In the liver the cannabis is metabolized to 11-hydroxy THC, that can be stronger then "regular" THC. The middle and inferior rectal veins drain directly into other blood vessels that go directly to the heart and to all the other body systems thus avoiding liver metabolism. This can be beneficial when using cannabis for anti- inflammatory and other reasons.
Perry Solomon, MD