Drivers like McLaren's Jenson Button and Ferrari's Fernando Alonso had questioned the value of testing at Silverstone, as a strictly-defined Pirelli programme limits what they can do.
Only half of the current F1 field will take part in the test.
Di Resta, whose Force India team is one of four squads running both its race drivers at Silverstone, reckons racers have more to offer as they competed in the British Grand Prix just a few weeks ago.
"There's value in every time you run a Formula 1 car," said di Resta, who completed his half day of testing on Wednesday.
"I think in terms of reference it's probably better that we are in the car, because we had the race here not so long ago, so we know where the car limits are and know what the differences are.
"I'm always keen to get in the car, keen to get driving, keen to help the team.
"That's what I'm employed to do and to give the relevant feedback that I can."
The Scot admitted testing was useful even with the limited things a team can do.
"Pirelli and the FIA are well aware of what we are here to do," he said. "We are not here to balance the car out perfectly.
"We are here to make sure we understand the tyres properly."