Relating to or involving benefits; producing beneficial results; of or pertaining to beneficence.
From beneficence + -ial (the suffix creating adjectives meaning 'relating to' or 'of the nature of'). A formation combining beneficence with -ial rather than using the standard -ent adjective form.
Beneficential is so rare that major dictionaries don't list it, yet it's perfectly logical—showing how English writers sometimes create needed words that never catch on because a simpler form (beneficial) already exists.
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