There are lots of different types of dementia, not just Alzheimers
The main advantage of knowing which type of dementia you have is in medication. Some drugs can only be given to a particular type of dementia (eg donepezil can only be used for Alzheimers) and other types of dementia cannot be given some medication (eg with Lewy Body Dementia there are quite a few drugs that you cant give)
If you have a diagnosis of dementia (any type) it is easier to get benefits - Attendance Allowance and then Council Tax disregard. There are usually dementia hubs or other activities which are designed for people with dementia and often offer support and information to their carers, but you cant access them without a diagnosis. As the dementia progresses, day care, respite and (ultimately) a care home may become necessary, but places that would be suitable will only accept people with a diagnosis.
I know all of this as OH does not have a diagnosis (long story) and I truly wish that he had.