I've never seen GTTs in production code, too. However, I often use them when I test my T-SQL code.

For example, there is a long-running stored procedure which returns a dataset, and I'm doing an optimized version of that procedure. I want to run a test to ensure that, given specified parameters, both procedures return the same dataset.

I open two SSMS windows, in one of them I run the following script:

CREATE TABLE ##result1 (...);

INSERT ##result1

EXEC dbo.Old_Procedure @param1 = value1, ...

In the other window, I run the following script:

CREATE TABLE ##result2 (...);

INSERT ##result2

EXEC dbo.New_Procedure @param1 = value1, ...

Both procedures execute in parallel, and I can compare the datasets without making junk permanent tables.