Functionalism takes as a base the use of first and simple materials. We can call it a subset of modernism because of the rejection of the idea of decorations, and it is also based on the 5 points of modernist architecture.
The building should reflect its use, its function. This type of construction was widely used because of its simplicity and ease of construction. Moreover, it is exported everywhere even in the US where the skyscrapers are the proud representation.