Healthful is a centuries-old adjective that traditionally means promoting good health. Over the years it has been pushed out by healthy, which traditionally applied to anything that was in good health (usually a person). For example, a healthy person would be one who eats lots of healthful foods. But today, most writers would use healthy for both the person and the good food. This is a long-established convention and is certainly not wrong.
Healthful is making a comeback, however, particularly in American and Canadian health writing (British and Australian writers have yet to go along). The change is not especially useful, however, because no one interprets a phrase like healthy snack to mean a snack in good health, so healthful‘s return is not preventing any confusion or ambiguity. Still, in good health and promoting good health are very different concepts, and having separate words for them is probably a good thing.