It's not just for unwrapping, it's also for animation and rendering as well. Not all quads must be triangulated though. Triangles are necessary when it comes to expressing a model's concave or convex profile, and also when you're optimizing, certain parts will have triangles.
Why Ngons are bad for unwrapping? Firstly, all the tools inside the UVW unwrapping will work and no errors will be shown whether you have all quads or Ngons, but with Ngons, you'll encounter distortion, skewed shells, tangled edges etc. and your UV will be a scrambled egg sometimes.
They are generally ok on Flat surfaces, but it's best to avoid it too. If you're exporting your model to another engine, then ngons are definitely not ok because different engines triangulate Ngons is their own unique way.
If you're doing this for an deforming animation, or for games, avoid ngons entirely.
