Linked from
The 8 pages that link to Factor theorem, each with the reason it gives.
Polynomial factorizationRelated: It connects finding a root directly to extracting a linear factor.
Polynomial rootRelated: It turns the condition that a value is a root into a factorization test.
Synthetic divisionRelated: A zero remainder in the procedure certifies that the divisor is a factor.
Cubic functionRelated: It turns a known cubic root into a linear factor.