Advance requests are made essentially because the patients know they are going to lose their ability to express their wishes one day or another because of their illness. And yes, it's for degenerative neurocognitive disorders.
However, other diagnoses may be involved. Let's, for example, look at Parkinson's disease, which is a degenerative neurological disorder, but can lead to cognitive problems. It's much less frequent in cases of amyotrophic lateral sclerosis and multiple sclerosis, which generally do not have a cognitive impact on patients.