One common analogy is to consider the genome sequence (a.k.a. assembly) as a map. Since the initial publication of the human genome in the early 2000s, most regions of human DNA has been known in full resolution. Other portions, most prominently the repetitive centromeres that lie at the middle of chromosomes, have remained unmapped. It was known that they exist, approximately how big they were, and which types of sequences lay inside, but the full order of the sequence had never been determined for any human genome until this work.
You could consider the genome like the earth and the centromeres like a dense rainforest. Previously we had detailed maps of most of the earth, and we had mapped the boundaries of the rainforest and had satellite-level images (i.e. we knew they were full of plants). Now we have on-the-ground pictures with full detail.
Having a map of these sequences makes the accessible to study. One of the most valuable uses of the human genome is as a shared coordinate system used by scientists to compare different individuals and identify and name genetic variants that explain human traits. We lacked that coordinate system for a big chunk of the genome until now.
As you say, this paper reports the sequence of a single human cell line named CHM13. Each of us has a slightly different genome sequence (really two of them, one from each parent). Now when scientists sequence the genomes of more individuals, they can look at these regions that were previously ignored. Certainly understanding those regions will improve our understanding of human biology. Exactly how much will remain to be seen.