Same value, different form
Simplifying 8/12 to 2/3 doesn't change the amount it represents at all — 8/12 and 2/3 are the exact same value, just written with smaller numbers. Converting 2/3 to a decimal (0.66666667, shown to 8 decimal places) changes it into a completely different kind of representation — a base-10 place-value number instead of a ratio of two integers.
Simplifying is exact; decimal conversion of a repeating fraction is not
2/3, simplified, stays exactly 2/3 forever — there's no rounding involved, since dividing both numbers by their greatest common factor is always exact. But 2/3 written as a decimal is 0.666666... repeating forever, so any decimal version that's actually displayed or typed — 0.67, 0.667, 0.66666667 — is necessarily a truncated approximation of the true value, not the value itself.
When you'd want each
Simplify a fraction when you need an exact answer for comparing or combining fractions — adding 1/6 and 1/6 gives 2/6, and simplifying tells you that's the same as 1/3, with no precision lost either way. Convert to a decimal when you need to plug the value into further arithmetic, sort several numbers by size, or compare it against a percentage or another decimal figure.
Round-tripping doesn't always get you back to where you started
Converting 0.75 to a fraction (3/4) and back to a decimal (0.75) returns exactly the number you started with, because 0.75 is a terminating decimal. But converting 1/3 to a decimal (typed as 0.333333333, nine 3s) and then that decimal back to a fraction gives 333333333/1000000000 — not 1/3. The round trip loses precision the moment a repeating decimal gets truncated to a finite number of typed digits; simplifying alone never has that problem, because it works with the exact numerator and denominator throughout.