Question

Why atomic mass unit is needed?

Answer 1

 atoms don't weigh very much, so common systems like pounds of kilograms would be very annoying to use. We use atomic mass units because they are on a useful scale for working with atoms; small numbers like 12 are easier to work with.

Answer 2

When you need to know comparative masses.   

for example, we have a unit calle the mole.   It is a specific number of atoms.   Well, if you need a mole of carbon, then you multiply the atomic mass of carbon times grams and you have ONE MOLE of carbon.  Terribly useful.   lets us do really useful chemistry.

Now, you could be asking WHAT is an atomic mass unit.  It is the mass of a carbon 12 divided by 12.  That means that generally speaking a proton or neutron has a mass of 1 AMU.   Weensy bit more for the neutron.  Thus the AVERAGE ATOMIC MASS of an element is the average mass (in AMU) of the isotopes of the element.   There is always more than one flavor of an isotope, so it is an average.  This means that if you need one mole, you can just multiply the "average atomic mass" times grams and you have one mole of that stubstance.   

We all know that one unit of lead would weigh more than one unit of aluminum, but this gives us the way to get exactly one unit of that stubstance.  Just look at the periodic table.