Step 1: Picture a diamond hierarchy where two parent classes both derive from a common grandparent. Normal inheritance duplicates that grandparent in the most derived class.
Step 2: That duplication is the problem: the grandparent's data appears twice, and calls to its members become ambiguous.
Step 3: Marking the common base as virtual instructs the compiler to merge those would-be duplicates into a single shared subobject.
Step 4: With one shared copy, the ambiguity disappears and the layout is clean. Enforcing pure virtuals, blocking overrides, and raw performance are not the goal here.
\[\boxed{\text{Only one copy of the base class exists}}\]