The reason Lord Jesus gave His faithful Church the Signs of the end leading up to His future return, is so His Church would NOT be deceived. He is not coming to gather His Church until AFTER the tribulation like He said (Matthew 24:29-31; Mark 13:24-27).
So He told His faithful Church how to stay in Him, and not be deceived, and that is what His command for us to WATCH is about. It is about 'watching' those SIGNS of the end He gave us (Matthew 24; Mark 13; Luke 21, which is His discourse on the 'end' while upon the Mount of Olives with His disciples; and the Signs of His Book of Revelation, which are the SAME Signs He was giving in His Olivet discourse.)
When Jesus said the following to the Church at Sardis...
Rev 3:3
3 Remember therefore how thou hast received and heard, and hold fast, and repent. If therefore thou shalt not watch, I will come on thee as a thief, and thou shalt not know what hour I will come upon thee.
KJV
... He was pointing to them how they (and us) are to be 'watching' to KNOW WHAT HOUR HE WILL COME.
When Jesus said in Matthew 24 that no man knows the day or hour of His future coming, but The Father only, then WHY... would He say the above in red to the Church at Sardis?
In Matthew 24:43, Jesus was saying the same thing, that if the goodman of the house had known in WHAT WATCH the thief would come, he would not allow his house to be broken into.
Matt 24:43-44
43 But know this, that if the goodman of the house had known in what watch the thief would come, he would have watched, and would not have suffered his house to be broken up.
44 Therefore be ye also ready: for in such an hour as ye think not the Son of man cometh.
KJV
And in Revelation 16:15, Lord Jesus tells us that He comes "as a thief".
It doesn't take an anvil to hit me on the head to grasp what He was teaching with all that. We are supposed to be WATCHING THOSE SIGNS HE GAVE LEADING UP TO HIS COMING. And by THAT... when the 'hour' gets closer, we will understand, and NOT be deceived! That is what His command for us to WATCH is about.
Thus the idea that He also said, that no man knows the day or hour of His coming, that is STILL TRUE, even with the above. But how?
It's because even with knowing and understanding those SIGNS He gave leading up to His coming, we still do not know exactly what day or hour that will be! If you understand the events about God's two witnesses in Revelation 11 on the 2nd Woe, when they rise signals the 3rd Woe coming quickly, which is the 7th Trumpet and shows the time of Jesus taking reign right then. We are told the two witnesses' dead bodies will be left unburied in the street of Jerusalem for 3.5 days, and then they rise. Thus Jesus by that gave us to within 3.5 days of His future coming!
But still, we cannot know exactly what DATE that will be. No man can know.
Now some men preach against the Sign's Jesus gave in Matthew 24 and Mark 13 for when He said He comes AFTER the tribulation to gather His saints. They abuse Jesus' command for His Church to WATCH, because they DO NOT TEACH THAT MEANS WATCHING THE SIGNS OF THE END HE GAVE. They just say, 'be ready', even though they wrongly preach to 'be ready' to be raptured 'at any moment', NOT PAYING ATTENTION TO THE SIGNS JESUS GAVE.
Thus not WATCHING means not knowing where we are in the timings Jesus gave for the end time events. One who does not WATCH JESUS' SIGNS might as well be living on another planet. And the danger of that mindset is this. These will NOT understand Jesus' Sign about the coming false-Messiah to Jerusalem to play GOD. It means they will be subject to the "strong delusion" that false-Messiah is going to work on earth with playing God. They simply WILL believe that first supernatural Messiah is our Lord Jesus! Thus these deceived that do not WATCH will be gathered to that false one like dead carcases to wheresoever the fowls are feasting on the carcase, per Jesus' warning of Luke 17:37 and Matthew 24:28.