before adding or subtracting two fractions they are converted into like fractions. Why?
We do this because it's easier for us to understand the addition/subtraction process, it simplifies the final answer, and thus allows us to do more with it.
Think of fractions like units. If you add 1 g + 1 kg, your final answer must only be one unit, so you have to convert gram to kilogram, or vice versa, so that your final answer is 1001 g or 1.001 kg.
A fraction like 4/3 ("four thirds") is 4 of something (thirds). If you want to add 5/6 ("five sixths"), you could just quit and say "I have four thirds and five sixths", but then you haven't really done anything. Instead, you can simplify by condensing into just one unit (13/6).
Now you can do more with this total. You can confidently say it's larger than 2/3 + 7/6, for example.