Why does a query execute so much faster when I (manually) cache the results of a table-valued function in a temporary table?

Why is Query Version 2 so much faster?

I suspect the DB Engine is calling the Table-Valued-Function "GetUsageStatistic" multiple times, so is there a way to tell the engine that "GetUsageStatistic" is deterministic and should be called only once?

Query Version 1

--Takes ~10 minutes
select *
from RosterLevel r
left join GetUsageStatistics( @mindate, @maxdate ) usage on r.UserID = usage.UserID;

Query Version 2

--Takes ~10 seconds
select * into #usage from  GetUsageStatistics( @mindate, @maxdate );
select *
from RosterLevel r
left join #usage on r.UserID = #usage.UserID;