They asks for signs and he first warns them to not be deceived and then lays out everything that will happen before his return. He gives them them the signs and later in in Mark 13 for example,
Mark 13:29 "So ye in like manner, when ye shall see these things come to pass, know that it is nigh, even at the doors.
How would one not know if these things have come to pass if they weren't watching for the signs he has just given? He lays it out perfectly so one is not deceived.
Later he states,
Mark 13:33 "Take ye heed, watch and pray: for ye know not when the time is.
Mark 13:34 "for the Son of man is as a man taking a far journey, who left his house, and gave authority to his servants, and to every man his work, and commanded the porter to watch."
Mark 13:35 "Watch, ye therefore: for ye know not when the master of the house cometh, at even, or at midnight, or at the cockcrowing, or in the morning:"
Mark 13:36 "Lest coming suddenly he find you sleeping."
Mark 13:37 "And what I say unto you I say unto all, Watch."
It's very important or Christ wouldn't have issued stricts warnings on this very subject as well as Paul later on. We shouldn't base our entire life on it but we should always be on watch and that should be an important part of our lives as Christians.