"It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge. " --Enrico Fermi

If you had knowledge of early-onset Parkinson's or Alzheimer's, you could plan for it by making sure to, say, spend much more time with your family than putting in lots of hours at the office with the expectation that you would be promoted later. You would also be able to manage some of them better with early knowledge and expectation of what is coming, and be able to volunteer for drug trials. (It may also affect your planning for cryogenics.)

If you had no knowledge of a neurodegenerative disease, it would be able to kill you anyway, but with no ability for your family to plan for it. You would similarly be unable to make choices about supporting research into that disease with enlightened self-interest. (Obviously, that may make little impact in the short-term, but if you learn in your twenties, then ~five decades of funding with a donation of a percentage of your yearly salary each year may very well lead to advancements in treatment).

Also, suicide is not inherently irrational.