Why does Jena have both Graph and Model?

Jena has two interfaces that represent two different representations of a set of triples.

The documentation of a Model says that an "An RDF model is a set of Statements" while a Graph is "The interface to be satisfied by implementations maintaining collections of RDF triples".

These don't sound very different to me. Why does Jena have both?

Good answers here on StackOverflow.