It's a question of ideology

1. There is a link between a rise in pandemic events and global warming. "Proving" it was a lab accident can be used as an argument that global warming had nothing to do with it (and of course, doesn't exist at all)

2. Blame: blaming a specific country could mean one could try to hold them liable for damages or at least set them in a bad diplomatic position. Plus when it's "man-made", it's easy to associate it with disliked people, like Fauci in the US pretending it was their personal fault. Holding individuals or institutions accountable/blaming them can also be a way to cope with loss.

3. Shifting the narrative: after millions had died, the narrative that COVID wasn't dangerous couldn't be used any longer. By claiming OT was lab-grown, there was a new story, a new scandal that would overshadow the fact that a lot of politicians, including the POTUS did not act and people can pretend they either never underestimated it or they did so because "a natural virus wouldn't be that bad". These narratives don't necessarily have to be for the public but for some just for their own conscience.

4. Theodicée: might be a bit abstract, but especially conservative americans are in the majority protestantic Christians believing in providence, i.e. that god makes all things natural come to pass. Meaning that a global pandemic would be gods will, meaning that the millions of americans who died would have been according to gods plan, maybe even as a punishment Not unlike the Great Plague in Europe was seen. A lab-grown virus produced by non-christians would be something god had no hand in, and it being "man made", would be a way out of the cognitive dissonance of god being good yet allowing bad things to happen to his devout followers.

Tl, dr: mostly it matters to blame others or explain how it could happen, also, of course as a political talking point.