The dative of purpose is often used together with the dative of reference- used to form a double dative. This is because datives of reference suggest advantage or disadvantage, so the dative of purpose provides additional information about the purpose of the advantage/disadvantage.
Datives of purpose and datives of reference do not have to go together by default- they can be by themselves.
Examples:
suis saluti fuit. (He was the salvation of his men.)
Cui bono? (translates literally to “to whom for an advantage” but is more commonly translated as “for whose advantage?”)