に (role/capacity)

The particle に marking the role, capacity, or function in which something serves — 'as X / for X's purpose.' 昼食に食べる ('eat as lunch'), プレゼントに買う ('buy as a present'), 記念に ('as a memento'), 言い訳に ('as an excuse'), 代わりに ('as a replacement'), 参考に ('for reference').