Because often, being in the now clouds your reasoning. It's the same reason you set goals and make investments; deferring instant pleasure will result in greater payment later, but you only understand this by looking at the sum of all parts.

By introducing the "deathbed" one assumes that the most important things to that individual come into focus. If I didn't have a family, I'd have different deathbed priorities, but that is not the case. If my greatest desire in life was to win a Nobel prize, I would probably make choices favouring that outcome.

I'm not saying one is better than the other, just that for most people, their families (especially kids) will be more important to them than any professional or individual achievement. In the daily grind that is life, we have a tenancy to forget this. Focusing on the sum of a life brings things back into perspective.

To see whether you optimized your expected utility throughout your life.