When we use fractions, we know that fraction is part of a whole. Therefore, we use fractions to indicate the approximate proportion of numbers.
E.g.
1/2. This fraction means that we divide into two parts and then we take one of them.
We use decimals when we don't have a whole number. Then, decimals can help us to calculate a number that is less than a whole number.
E.g.
4.6 - This means that the value has 4 units and 6 fractional parts.
We use fractions because are easier to understand with part of 100. Percentages are simplified ways of expression sizes or values.
E.g.
20% - This means 20 parts of every 100.