Why are ToLookup and GroupBy different?

Question

.ToLookup returns an ILookup. ILookup also implements interface I

Answer 1

why would I ever bother with GroupBy? Why should it exist?

What happens when you call ToLookup on an object representing a remote database table with a billion rows in it?

The billion rows are sent over the wire, and you build the lookup table locally.

What happens when you call GroupBy on such an object?

A query object is built; end of story.

When that query object is enumerated then the analysis of the table is done on the database server and the grouped results are sent back on demand a few at a time.

Logically they are the same thing but the performance implications of each are completely different. Calling ToLookup means I want a cache of the entire thing right now organized by group. Calling GroupBy means "I am building an object to represent the question 'what would these things look like if I organized them by group?'"