People wanted to describe relations of two integer,
If you have a line which his length is 1/3 so it's decimal form is 0.333333... infintly. but if you want to draw 1/3 of 1 cm it is not possible because you cant keep adding 0.3+0.03+0.003 to infinity... but I obviously can make 1/3 of a cm rope. so I have to describe the length by a relation of two integer. And I can fold the rope into 3 parts of 1/3 cm.
