Here's a bit of a different take, based on critical scholarship (not my own).
Referred to in many critical circles as the Eschatological Discourse, it is agreed upon by scholars that each of the Synoptics employ it in different manners, each with a slightly different purpose.
The Marcan version is provoked by Jesus' disinterest in the Herodian edifices with the questions of the apostles due perhaps to a line of Jewish belief that the Temple would fall only in the end of days. The discourse occurs in the final section of Mark which deals with "revealing" the reason for the Messiah's coming, which according to the Marcan version is summed up in the events of the Passion. This makes the discourse part of the revelation of who Christ truly is, but it offers little beyond advocating a constant need for watchfulness in the end.
Matthew's account (though believed to use Mark's as a source) does not occur as part of the Passion. Instead it acts as the finale to the ministry in Judea and Jerusalem. While in Mark the discourse centers mainly on the Temple due to the likely elementary belief of its survival until the end of history, Matthew's account seems to divide the two events more carefully into 1.) the destruction of the Temple, and 2.) the end of the age. In Matthew's account the demise of the Temple is typical of history's end, but the details about this end coinciding with the Parousia are described in language borrowed from Hebrew apocalypse, such as found in Daniel. The account ends with two parables which may or may not have been a part of the actual Olivet discourse. Matthew is well known for combining like material categorically over chronological order, so this is possible. The lack of connection with Mark's glorified Jesus and the parable conclusions suggest that an exegetical approach similar to apocalyptic deciphering was perhaps already being employed by this gospel's composition instead of expecting a literal Parousia of Christ being connected to the historical fall of the Temple (which had likely already occurred by Matthew's composition).
The Lucan account is more like Mark's. The discourse is spurred on by the same and gives a similarly combined answer using the Second Temple's demise as a sounding board. But the "signs" of the end are interpreted more as symptoms of everyday life that are taken advantage of only by the false prophets and are set as a warning against being deceived into believing that Christ has returned. It ends with the Marcan view that actual interpretation is not as important as being vigilant at all times. Luke also ends Jesus' ministry with this discourse, as does Matthew, separating it from his account of the Passion.
While different schools of thought exist, mainstream eschatology in Catholic, Anglican, Lutheran, Methodist, and Presbyterian exegesis favors none of the approaches that suggest a literal interpretation of history's end via this discourse. As the Marcan and Lucan endings suggest, vigilance is viewed as the final requiste taught by these accounts.
Matthew's discourse suggests an even richer moral lesson, adding two parables to remind disciples not only of the importance of vigilance but of living out their discipleship each and every way through active participation in God's redemption of the poor, stating that their salvation would depend not on interpreting the "signs" of the discourse but finding Christ "in disguise" as the poor and marginalized who need to be ministered to.