I you add 10kg to your weight, your avatar will move more slowly, especially uphill. Putting out, say, 200w will/should feel like putting out 200w irrespective of your weight or speed.
I can see the point of loading up your panniers with bricks and riding IRL to accustomise yourself to the handling of the bike and align your expectations of speed, but there is zero value of doing so in, say, Zwift.