This article touches on something that remains very true today: if you want to study genetic disorders you look at closely-related populations. If the condition you want to study is linked to a recessive gene, you want to find (or create) an isolated population with that gene.
In animals/plants this is why inbreeding can serve a useful function. If you want to detect a recessive gene, but either don't have access to genetic testing or are not sure which gene is involved, you in-breed until you see individuals who express the recessive gene. These individuals inherited the recessive gene from both parents. You then know which parental lines carry the gene, which lets you select for or against it.
My country of origin, Faroe Islands, has worked on this precisely because the population is small and very closely related. So there's a lot of recessive genes being expressed. Also, they've kept meticulous population records for centuries, so traits can be accurately traced back through several generations. I just looked up my own records in the national registry and traced one of my branches back to a named person born in the year 1170.
Many, many species of plants are inbreeders. In the plant world, inbreeding and outbreeding seem to both be successful techniques with their own benefits and drawbacks.
Inbreeding gets a bad reputation in humans because we have poor genetic diversity and endless recessive diseases due to going through at least two near-extinction events (population bottlenecks) that took us down to 2000 and 55000 humans.
https://gizmodo.com/extinction-events-that-almost-wiped-out-...
Many animals can inbreed with much less downside.
There are lab mice that are entirely homozygous (inbred until every gene they have is identical). They're not quite as prolific as normal mice, but they survive.
In animals/plants this is why inbreeding can serve a useful function. If you want to detect a recessive gene, but either don't have access to genetic testing or are not sure which gene is involved, you in-breed until you see individuals who express the recessive gene. These individuals inherited the recessive gene from both parents. You then know which parental lines carry the gene, which lets you select for or against it.