Decimals are more precise than integers.

Unless you are a computer programmer or an electronic hardware engineer, every number you encounter in daily life is a decimal number. We commonly encounter numbers when talking about time or money, travel distance or speed, and measurements or dimensions of any sort. You also encounter decimal numbers when tuning your radio or television, or when selecting tracks on your CDs. Video game scores are commonly reported using decimal numbers.

To make the number or answer more precise.

It gives us the exact amount, no more no less.

They're used to accurately calculate numbers into percentages and a fraction thereof of the same or a portion of a whole

No idea... I think they are pointless... But I suppose in certain jobs they could be very useful to master