Why do Lapidaria margaretae look like stones?
"Teleology is like a mistress to a biologist: he cannot live without her but he's unwilling to be seen with her in public." - J.B.S. Haldane (1892 - 1964)

I can't tell you why Lapidaria margaretae looks like 'stones'. But I can deduce something from the way this plant looks. 

First and foremost, I can say this plant probably evolved in an arid region, possibly a desert (less than 10 inches of water per year), because it is a succulent, with a shape that comes pretty close to minimizing it's surface to volume area (decreased evaporation, decreased transpiration). Seeing as it's a dwarf and shaped as it is, I might hazard that it's windy and sandy where this plant evolved, also because of it's shape - low to the ground, sturdy, low surface area, able to resist the wind. Finally, the light color of the plant suggests a lot of bright sun (reflects sunlight more than some darker colors).

None of that is foolproof, but in this case, it turns out to be right; the plant comes from southern Namibia, and is commonly called the "Karoo rose".

This is one image of the Karoo region:



Stones come in all kinds of shapes and sizes, also influenced by water, wind and other environmental influences. That this plant looks like stones is mostly coincidental.