Basically, linear relations is put the results were they should be.This results should be always a pattern, then we can identify if the pattern is decreasing or not with a visual resource. It also is very useful for any job that we should take.
Is the number that can be written in a fraction. That is extremely important for the academic life and either more for the adult life. In things all over our life we will have to calculate things [especially money] and devid numbers as a fraction is just essential.
Another essential thing for the business and students lives. This is usually used to calculate not specific answers, but it helps a lot. Calculate the minimum price for something is just a common example for the use of it.