To:
1. Diversify their portfolio and speculate on bitcoin price... millionaires are usually looking to make their money grow, and this is one of the most attractive speculative vehicles at the moment.
2. Protect themselves from currency inflation... if you're a Chinese millionaire I think it's worth holding a bit of your wealth in bitcoin
3. To escape capital controls, or hide/protect your money... you've heard of how much $$ was hidden in offshore accounts revealed by the Panama papers right? There is demand for these offshore accounts, and bitcoin is another use case for this used properly
Shopping with bitcoin is an utility that will become convenient later, right now it's not imo, but greater adoption and acceptance, the implementation of segwit/LN, more platforms, etc will make the experience better and give people greater incentive use bitcoin over cash/credit card. Right now I think shopping via bitcoin is one of the least important use cases for bitcoin, which newbies typically think is all bitcoin is used for.