The donation of a brain can be valuable to researchers working on neurological causes of diseases.
For many diseases, one of the important things scientists need to continue their research is something you might have and it is not money. Scientists might be able to use your brain after you pass away.
There are several common diseases that change the structure of a person's brain.
Researchers can use the organs from people who suffered from those diseases to help others who will suffer from them in the future. While some of those diseases, such as Alzheimer's and Parkinson's, are well-known by the general public to have neurological causes, the neurological causes of other diseases are not as commonly understood. For example, depression and post-traumatic stress disorder are often assumed by the general public to be psychological, but they are actually neurological and can change the structure of the brain.
Reference: BBC (Feb. 17, 2017) "Scientists appeal for more people to donate their brains."