I enjoy talking to experienced older masons.
I remember one guy in particular, who took an interest in my triangular block system. He was thinking about mortar, and shared this with me: “Mortar does two things. It keeps blocks together and it keeps them apart.”
Mortar glues concrete blocks together, and it also keeps them from touching each other. When blocks touch each other, they are more susceptible to cracking. Mortar acts as a damper between blocks.
It keeps them together and keeps them apart.