Metol is soluble in concentrated sulfite solutions. People do it every day when they mix up Kodak single package D-76. Remember in this case everything goes in at the same time. Metol just dissolves more slowly and requires more stirring. Kodak also recommends warm (125F) water. Without the small addition of sulfite the amount of metol that is oxidized is insignificant. When making up D-23, I have without thinking dissolved the sulfite first and can say that with heat and a magnetic stirrer to do the work that Metol will dissolve eventually. So the reason is mainly speed of dissolution.
Developers which also contain hydroquinone in addition to Metol can precipitate the addition product of these two developing agents. I routinely mix up a concentrated developer which forms a slurry with the addition of the hydroquinone. The adduct dissulves when the alkali is then added.