Mitchell if you haven't got anything new to test it's a waste of time.
With all FPR's well publicised problems ,cars not ready, shifting buildings, at one stage they didn't even have enough engines to do a test day.
The Grand Prix is a great thing for the V8's, it allows the teams to see if anybody has made any gains during the off season.
Most teams treat this as test days anyway.
Nearly all the teams debut new cars just before the endurance races, not at the start of the season, so testing before the season starts really has no advantage and just uses up one of those precious test days