Genki II explains that it should be used to show that you did somebody a favor (just like あげる), but particularly when the main verb of the sentence does not have the place for a beneficiary to receive your favor. An example sentence they give is 私はゆいさんのために買い物に行きました。
I’m not understanding the point of this. Couldn’t I just say （私は）ゆいさんに買い物に行ってあげました? How is there not a place for a beneficiary in this sentence? Is the sentence I wrote grammatically incorrect?
3 comments
Well, ゆいさんに買い物に行ってあげる sounds like you’re telling the listener you’ll go to a shop called ゆいさん for them. You can only use just に with ～てあげる if the original verb already uses に that way.
ため is also a lot wider in use and not limited to a person as beneficary.
家族のために転職した
= I changed jobs for the sake of my family.
e.g. the reason I changed jobs might be that it meant I could spend more time with my family, or because it let me move to a place with better schools for my children, etc, etc.
仕事のために英語を勉強している
＝ Studying English for the purpose of work (as compared to out of personal interest),
ゆいさんのために買い物に行った is factual or neutral.
ゆいさんに買い物に行ってあげた is ungrammatical.
ゆいさんのために買い物に行ってあげた sounds patronizing her.