What is the point of Lookup<TKey, TElement>?

The MSDN explains Lookup like this:

A Lookup<TKey, TElement> resembles a Dictionary<TKey, TValue>. The difference is that a Dictionary<TKey, TValue> maps keys to single values, whereas a Lookup<TKey, TElement> maps keys to collections of values.

I don't find that explanation particularly helpful. What is Lookup used for?