Financial independence, of course.
In divorce papers signed in 1919, which finally dissolved Einstein's troubled marriage to his first wife, Mileva Maric, the theoretical physicist left all his Nobel money to Maric and their two sons. There has been a lot of speculation around that decision. Some have suggested that Einstein felt indebted to Maric — it has been rumored that she, herself a budding young scientist, helped author some of Einstein's most famous work. Although there's no clear evidence that she co-wrote any of his papers, few historians doubt that she assisted her husband and often provided him a sounding board.
Perhaps more intriguing is Einstein's bold prescience:
He left the money to Maric in 1919 (in a notarized document, no less), yet was not awarded the Nobel Prize in Physics until 1921. — R.F.
Podcast: A Brief History of the Nobel Prizes