The point is right now the doctor has to just guess whether it's Alzheimer's or something else unless they can do expensive imaging tests and a spinal tap with expensive testing on the spinal fluid at a reference lab. An antibody test on blood would be way cheaper and easier to do at scale and allow earlier and more confident treatment to more people. This will become more important as treatment improves over time (hopefully).

Many people go "dementia they must have Alzheimer's" while that's pretty commonly true it isn't how it works. Dementia is merely a symptom but dementia has many other causes. It can be beneficial to make sure the symptoms are from Alzheimer's and not something else to ensure proper treatment. Plus I see this as one more step to understanding Alzheimer's since we scarily don't really know much yet it's about 1 in 5 will get it depending on how long they live and such. If we can understand the disease we can find better ways to treat it since treatment currently doesn't really do anything. The scary thing about Alzheimer's too is that for treatment to really make a difference on someone's life you need to start it as soon as the disease starts which is decades before others observe it's happening. We only realize the person has Alzheimer's when it's in a progressed state. In reality the disease probably started as early as their 30s and 40s and people just thought "you start forgetting as you age" which is true but it's also an early sign of the disease. So for me I know Alzheimer's runs in my family. Almost all of my grandmas 16 siblings got Alzheimer's. If I can get tested every other year starting at 30 or when I start noticing a decline in my memory I can get the treatment we currently have and prolong how long it'll be until the symptoms affect my every day life.

It's good that your mother had the support structure of other folks to notice the changes, not everyone is so lucky.