You omitted chicken and fish as sources too.
Gelatin is a dense protein for a start. It contains the following amino acids - Proline , choline, glycene, and glutamine that are essential in maintaining and synthesising collagen in our bodies. They also help protect muscle.snd joint maintenance. It helps reduce osteoarthtitis and osteoporosis. It is not just a cheap filler, and many vegetarians suffer deficiencies by avoiding it.
My grandparents were bought up to eat the bone marrow from the sunday joint and had specisl spoons to extract it. They also brewed beef broth from the caracass bones, and there were jars of beef or chicken extract available from the chemist to assist patients recover from illness. Not the same as marmite or vegemite at all. You do not get any of this from a supermarket shelf and is one of the aspects of the Old Times that may have actually made sense.