Fractions, decimals, and the same number wearing two hats
A fraction like three-quarters is a division that has not been performed yet: the numerator counts parts, the denominator says how many of those parts make a whole. A decimal is the result of performing that division in base ten. The two are not different kinds of numbers — they are two notations for the same rational number, and every calculator in this group converts between them or operates on them.
Which notation you should use depends on what happens inside the denominator. A fraction simplifies to a terminating decimal exactly when the reduced denominator contains no prime factors other than 2 and 5 — the prime factors of our base, ten. So three-quarters (denominator 4 = 2 x 2) becomes 0.75 and stops, while one-seventh becomes 0.142857142857... and repeats forever, because 7 is not a factor of ten no matter how many zeros you attach. Repeating decimals are not imprecise numbers; they are exact numbers in a notation that hides the repetition.
This is why the exact fraction is the safer working form. Adding one-third and one-sixth as decimals invites rounding, but as fractions the common denominator of 6 gives two-sixths plus one-sixth — exactly one-half, no rounding error anywhere. Convert to decimals only at the end, and round only once.