Units help us to determine the physical quantity easily.
You want to write a velocity of 5 m/s.
But you wrote 5.
Now i see it as 5. How will i come to know that its velocity?
Now you will say that i will write Velocity before it.
So consider this questions,
A body moving with
A body moving with 5 and 25 travels 25. find the Time required. .
Now you might have understood.