Level freezing was a thing when the best way to get advanced refine runes was through sage. Since epic refine came out and advance refine runes are much easier to get now, I don't really see much purpose to permanently level freezing. This doesn't mean you should rush through levels. If you are struggling to beat the storyline missions, strong approaching, or wanted missions with your power at your level, then you might level freezing for a while to get your power up.