Nice exercise facilities & cafeteria, substantial per diem, and the best healthcare your money can buy.
Not to mention establishing wealth from connections. Many of them come to DC with little financial wealth, and leave with much financial wealth.
Or from the time they serve, have connections to soon obtain financial wealth as lobbyists, consultants, board members etc.
The Clinton Foundation is simply a larger scale "opportunity" compared with arrangements many have.
I have seen this on local scale. People serve on city councils, then get cushy positions with monopolies like trash collection firms, as one example.
Obama is staying in DC, for another example. His wealth has and will multiply rapidly.