1) Their sizes are easier to compare. It's easy to see that 5.2 is less than 5.4, but comparing 6/54 to 13/52 takes a little more work.
2) Most physical objects are easier to measure as a decimal approximation. It might be easier to measure a piece of wood that's 0.21875 feet instead of 7/32 feet.
3) Decimals are expressed in fractions of powers of 10, and we use powers of 10 in our number system. Dividing 345 by 100 is easy to express as 3.45 rather than try to think of it as a fraction
