External optics are a means of transporting a laser beam from one location to another in a form required at the other end.
Here are some quick examples:
Collimation: adaptive optics, fiber coupling, controlling divergence
Beam expansion: Illumination, holography
Beam contraction: cutting, marking, etching, beam waist control
Beam splitting: interferometry, holography
Beam combining: light shows
These are just a few examples of the thousands of uses of optics in laser systems. Unless you want to try out some of these things you really have no need for external optics.