Why does D-Bus use a variant type?

The variant type (v) exists because D-Bus's type system is otherwise strict and static: every message's signature must be known ahead of time, which is awkward for APIs like properties, where different properties on the same object can have completely different types.

A variant wraps a value together with its own type signature inside a single slot, so a method or property can declare its argument as "a variant" without committing to what concrete type is inside until runtime. This is exactly why org.freedesktop.DBus.Properties.GetAll returns a dictionary of a{sv}, string keys mapped to variants, since one property might be a string, another a boolean, another an array, all returned through the same generic structure.

The trade-off is that code consuming a variant has to check its inner type at runtime before using it, since the outer signature alone doesn't tell you what's inside, unlike a plain s or i argument whose type is fixed and known in advance.