If you’ve been gardening for any length of time, a multitude of these routines accumulate after a while. Often they are based on research, or on the little booklet that came with the plant, or because your dad always did it that way. Follow the same maintenance care for a plant–or a garden–for a decade or so, and I guarantee the “why” of what you are doing, at least for some things, will fade into oblivion.
According to Rayford Reddell, as alfalfa decomposes it yields triacontanol, which acts as a growth stimulant, encouraging basal breaks. Basal breaks is rosarian-speak for “increased vigor and flower production.” Now, I know I didn’t know that before, but I’m glad I found out.