Why do many concrete blocks have this strange shape on either end?
As with most building materials, there is a balance between strength, weight, cost and other attributes.
Solid concrete blocks would weigh roughly twice as much as these blocks and cost twice as much too. But they wouldn't add that much strength. In addition, the hollow sections make it easy to grip, move and place the blocks, whether by man or machine.
With the blocks stacked in a wall so that the hollow areas are vertical or horizontal, the visible faces of the wall are solid. With the hollow areas vertical, they can be filled with concrete and rebar for additional strength, particularly including anchoring to a foundation: rebar embedded in the slab poking up into where the blocks are placed and then filled with concrete. Here is one example article explaining how to do this.
As far as the &quot;I&quot; shape, that little gap in the middle of each end allows for mortar to hold the blocks together while still having the faces of the wall &quot;just blocks&quot;.