And why did he tell us to watch for it?
Jesus did not tell us to watch for the Last Days.
He told His "generation" to watch for the destruction of Jerusalem, which was to occur within the lifetime of His listeners.
Matthew 24:34
Truly I tell you, this generation will certainly not pass away until all these things have happened.
And His followers heeded His warning and fled Jerusalem, escaping to Pella.
Yes, the Olivet Discourse is hard to understand, because it contains prophecies for both the immediate and distant future, some of it told via symbolism and allegories.