In some systems, such as five-card major systems, it is necessary to open in a minor suit with fewer than four cards. Playing the ‘better minor’ method, a minor-suit opening always guarantees at least three cards. Players who open 1♣ when they have 3-3 in the minors are not actually playing ‘better minor’, though they often use the term incorrectly for such a method.
If you are playing five-card majors and a strong no-trump, you cannot open 1♥ (or 1♠) as that would show five. Nor can you open 1NT as that would show 15-17. The solution is to open 1♦, better minor.