In addition to cables and backplanes that have been mentioned, the entire path from RAM to PCIe to disk controller has non zero risks of errors. That said I'm pretty sure that the BTRFS errors I've seen from SSDs have all been the fault of the SSDs in question.

Also note that hard drives have had internal error detection and correction codes for decades, so SSDs doesn't change things much. Probably most checksum errors are misplaced writes and missing writes.

HDDs also use ECC hence why read errors occur on failing drives instead of just silently returning corrupt data. That's not an ironclad guarantee and plenty of places in the data path can let an errant cosmic ray flip bits. BTRFS doesn't even eliminate this, it just reduces the surface area and most importantly gives you a way to durably detect or correct this from when the checksum is generated in the kernel onwards.

Errors can happen during transmission as well, especially if cables or backplanes are involved.

It's just as useful on either. Error correction predates SSDs.