Machines used for Worker Nodes

Hello all,

I was reading about machine sets, and the following was stated:
"Machines used for worker nodes should come from a machine set."
Keying on the word "should" suggests to me that this is not a "must" (i.e. it is not a requirement).

My question is, why "should" machines, used for worker nodes, come from a machine set? Is it possible for machines, that use worker nodes, to be generated (come from) by some other means?

Thanks in advance.