Roosevelt's "New Deal" of 1933-1938 was the most comprehensive overhaul of federal intervention ever seen in the US, and was unparalleled thereafter. In order to pass many of his laws within the deal itself he expanded the supreme court from 6 judges to 9, the three extra owing loyalty to him.
So basically he expanded the justices so he pass his bills.
Hope I helped, and please rate my answer :)
