Question

Why might suet be particularly suitable for an old man to eat?

Answer 1

Because it is a fat and would add fat to the diet.   It can be used several ways.

Answer 2

Suet is fat. Fat gives energy to human diet.

After 60 years immune system of old people becomes weaker, therefore they have to follow balanced diet (good nutrition) to gain enough energy for the day. This necessity covers suet, but in little proportions as in big portions it can lead to heart attacks.