Fay,

We don't always! But when we do it might be because...

*decimals are easier to compare:

is 8/13 bigger than 14/23?
is 0.61538... bigger than 0.60869...?

*decimals are easy to approximate; if you only need to be within 1% for a practical purpose you just round. You can do this with fractions but it's trickier.

*decimals are easy to add (fractions are often easier to multiply.)

Good Hunting!
RD