。
Mother wrapped a sandwich in paper for me.
Literal
Mother [topic-は] sandwich [obj-を] paper [in-に] wrapped-for-me.
The auxiliary ~てくれる encodes the benefactive relationship: mother did the wrapping, and the benefit flows to the speaker. English needs an explicit 'for me' to convey this; Japanese bakes it into the verb ending itself. Without くれた, the sentence would simply report an action; with it, the speaker's gratitude and perspective are built in.