Strictly structured math documents often contain certain numbered textual elements, which can be definitions, theorems, lemmas, examples remarks, and so on. Numbering is mainly for cross-referencing. Such structuring is especially the case in self-contained scientific works such as theses.
As a sample application, we will create a definition, a theorem with a proof, a lemma, and an additional note. All shall be automatically numbered for cross-referencing. Just for fun, we will apply it to school geometry: the theorem of Pythagoras.
Actually, this recipe is of pretty universal use: you can create numbered, named environments, ready for cross-referencing, whatever it shall contain at the end.