How can it make a difference to me if I'm dead? Or if I get Alzheimer's? In that case you are just deceiving yourself thinking you can have some purpose - a temporary one at best, but when you die you're dead.
Imagine someone is in the middle of a desert with no one there in the circle of 2000 miles, and has only 0,1l of water. Without that water the person died in 3 days. With the water, the person died in 5 days. Did that water make a difference, had a very important purpose?
If you didn't get it already I don't think you're going to get it... what you do in life has an effect upon yourself when you're alive and upon others while you're alive and even when you're dead.
Your allegory is twisted, because two days in the same gruesome conditions are of course no difference (except if they get him rescued). But what if Einstein had said at age 25 "look, when I'm dead I'm dead, there is no purpose to all this... so I might as well just sit back and sulk my life away"? He wouldn't have had such a fascinating life and wouldn't have left the legacy he left.
Why is there the need for a cosmic significance for life to have a purpose? Millions of non-believers have had extremely fulfilling and thrilling lives full of sense and purpose.