The first parable is an introduction to those that follow, and the last one is a conclusion and application of the whole series. [1][source]
"Modern interpretation of the parable has increasingly recognized this implication of the literary form of this particular parable, over against the dogmatic assertion of earlier NT scholarship, following Adolf Jlicher, that a parable has only a single point and that all the rest is mere narrative scenery, which must not be "allegorized" to determine what each detail means. In this cast the way the story is constructed demands that the detail be noticed, and to interpret those details individually is not arbitrary "allegorization" but a responsible recognition of the way Jesus constructed the story." [2][source]