Additionally is from additional plus -ly. The suffix -ly recasts additional as an adverb: the lexical idea remains recognizable, but the word now describes manner, degree, or circumstance rather than naming the quality by itself. In use as an adverb, additionally denotes used to add more information; also. Typical contexts include “The hotel is cheap, and additionally, it is close to the beach”. The meaning stays close to the form: it applies the etymological material to used to add more information; also, keeping the visible parts legible in ordinary use.