Ordering a set of numbers from least to greatest (or vice versa) requires comparing and understanding relative magnitude. This reinforces place value and builds number sense.
You already know how to compare two numbers and say which one is bigger or smaller. Ordering takes that same skill and applies it to a whole group of numbers at once — instead of just deciding which of two numbers wins, you're putting a whole list in the right sequence.
Think about lining up for a water slide. Everyone has a height number, and you need to arrange the line from shortest to tallest. You look at two people, decide who's shorter, and find their place in line. Then you look at the next person and find their place. You repeat this until everyone is lined up in order. That's exactly what ordering numbers means: comparing, deciding, placing.
When you order numbers from least to greatest, you're building a staircase going up — each number is a little bigger than the one before it. If your numbers are 14, 7, 19, and 3, you start by finding the smallest (3), then the next smallest (7), then 14, then 19. From least to greatest: 3, 7, 14, 19. Greatest to least is just the same staircase walked backwards: 19, 14, 7, 3.
Here's a useful trick for numbers up to 20: the number line in your mind is like a ruler that already has all the numbers in order. If you can picture the number line from 0 to 20, any set of numbers you need to order is already arranged on it — you just need to read off the numbers you're looking for from left to right (least to greatest) or right to left (greatest to least). As you practice ordering numbers, you're building this mental number line stronger and stronger, and it will help you with everything that comes next — like adding and subtracting in your head.