To expand this a bit, most sequencing methods are exact, and have a low error rate (except nanopores).

But they produce short reads, and because DNA is full of repetitive fragments, it's not always clear where the read came from.

We also have two copies of genes, which also further complicates matters.

The first startup where I worked, developed synthetic long reads on top of Illumina's hardware. We could stitch together 50kbp reads, which really helped with de-novo sequencing.