Denrai are heirloom items that belonged to well known clans from the feudal era of Japan.
Some of these were very powerful regional clans and have famous warlords in their lineages, and many of them played significant roles in the history of Japan. These are clans like the Uesugi, Shimazu, Mori, and so forth.
There are also minor clans without significant power bases. On top of this all we add the Tokugawa Shogun and the Mito, Kishu and Owari branches of the Tokugawa family who stand apart from the other daimyo.
There are some reasons why denrai status is important, outside of the historical interest and coolness factor to have a sword that belonged to one of the major clans.
When we look at the NBTHK Juyo zufu there are about 453 out of almost 12,000 swords that went Juyo and higher that have preserved information about what clan handed them down. This is a surprisingly small number of only 3 percent and points out that it is quite rare to have one of these blades.