A thesaurus is NOT for poetry, says poet Mark Doty:

If you write a poem with the aid of a thesaurus, you will almost inevitably look like a person wearing clothing chosen by someone else. I am not sure that a poet should even own one of the damn things.

This Lapham's Quarterly essay says what a thesaurus is actually useful for:

A thesaurus, as we have seen, can mine rich usage data from textual corpora to paint a picture of how words are used in actual context. It can create new spatial metaphors for semantic connections. Or it can add a historical dimension to trace how words related to a given concept have ebbed and flowed over the centuries.

Contextualizing. Connecting. Adding historical dimension.

There you have it.

While I'm asking, what's another word for thesaurus?

Thesaurus's are great for university.  So you can understand what the hell lectures are saying, and so you can paraphrase people.

Good points, Estelle, although even with a thesaurus I can't understand what lectures are saying!