PHP has several callable syntaxes — a string, an array of object and method, an invokable — and functions accepting callable have to tolerate all of them. Converting once at the boundary makes everything downstream a real closure.
$fn = Closure::fromCallable('strtolower');
$fn = Closure::fromCallable([$this, 'normalise']);
// and it respects scope, so a private method becomes callable
$fn = Closure::fromCallable([$this, 'privateHelper']);The scope point is the one worth knowing: the closure is bound to the scope where fromCallable was called, so a private method converted inside its own class stays callable afterwards from anywhere the closure is passed. That is genuinely useful for a collection pipeline and genuinely a way to leak internals if used carelessly.