This is something you'd learn in your first year at law school. If, as Trump's complaint alleges, Dominion did destroy evidence then the evidence is immediately assumed to support the opposing party's allegations.
Biden's defense would now have to prove there was no fraud as if Trump's team had certain evidence of there being such.
When you move past law school and into practice, you learn that destruction of evidence is defended by inadvertent accidents and other procedures put in place before litigation came to light.
It sucks and especially when the govt destroys the evidence.