If it were going to rain at any moment, how long would you wait before you went out and collected the clothes drying on the clothesline?
Well, that depends on the number of clothes on the clothesline, you might say. If there were a handful, you could pick them off quickly! Sure enough, but what if there are more clothes on the line and the line is long, or there are more than two clotheslines? You would of course go out earlier so that you manage to get every single piece before it began to rain; and if it did begin to rain while you were at it, you might attempt to pick off the farthest ones first or the dry ones, based on your decision at the time.
Planning and work prioritisation go together. You plan ahead, based on the number of jobs or tasks due. Completing harder ones or easier ones is a choice that you make, based on your understanding of the situation and the time remaining. To avoid holding too many pieces - should some of the clothes fall to the ground - you might also consider collecting the clothes at two separate intervals, perhaps the ones that are already dry first, leaving you with a lesser number to collect, if the rains came down. Likewise, at the workplace, completing tasks while you can, reduces the last-minute pressure.
Planning is a one time effort. Prioritisation happens constantly.