As it was in the days of Noah

i) I'm going to make a few brief observations about the Olivet Discourse. I'm not going to discuss all the exegetical twists and turns of this complex text.

I take the traditional position that this refers to two distinct events: the fall of Jerusalem and the return of Christ. I view the former as past and the latter as future.

ii) One objection to the traditional interpretation is that it allegedly inserts a large temporal gap between the two events. To that objection I'd say several things:

a) It's inaccurate to say the traditional interpretation inserts a temporal gap. Rather, the text itself is indefinite on the duration of the interval. 

b) Apropos (b), at the time Jesus spoke, both events were future in relation to the disciples. And assuming a pre-70 date for the Gospels, both events were still future in relation to the original reader. 

But obviously there's a shift in the viewpoint of a modern reader. At least one of the events is past in relation to the modern reader. So there's a sense in which we're bound to see it somewhat differently than the original audience. 

c) There's an unspecified interval between Christ's prediction and the fall of Jerusalem. The disciples had no idea how soon that would take place. To make allowance for a temporal gap between the fall of Jerusalem and the return of Christ is no more ad hoc than making allowance for a temporal gap between the prediction of Jerusalem's downfall and the fulfillment. One way or another, the disciples, the original reader, and later readers must all take a wait-and-see attitude. We find out when they will happen after they happen. 

d) The objection to a temporal gap presumes that if, in fact, there were such a gap Jesus or the Gospel writer would give some indication, perhaps by filling the gap with intervening events.

However, ever so many things happened in the decades between the prediction and the fall of Jerusalem which Jesus and/or the Gospel writers don't bother to detail. If, therefore, there was a gap between the fall of Jerusalem and the return of Christ, there's no reason to expect Christ or the Gospel writers to spell out a series of intervening events. 

e) This also goes to the nature of Biblical priorities. From a theological or eschatological perspective, after the fall of Jerusalem, what's the next big event? Sure, lots of things may happen between then and now–things which you and I may think are important–but do they rise to the level of the next big event? If we're waiting for the coin to drop, that's the Parousia. Nothing in-between measures up.

iii) Why do the Gospel writers record both predictions? How does the general reader benefit from having that information? 

Let's put if this way: why should the reader believe Christ's prediction about the end of the world? Well, for one thing, because he accurately predicted the fall of Jerusalem. 

In fact, in the Olivet Discourse itself, we have a similar principle concerning Noah's flood, where a past event sets the precedent for a future event (Mt 24:37-39). Likewise, Christ's ability to predict the fall of Jerusalem attests his ability to predict the end of the world. If the former came to pass, we can expect the latter to eventuate as well. We can't directly verify the future. But if he made a verifiable prediction about what is now a past event, then that corroborates his foreknowledge.