Final answer:
Institutionalists use economic and political approaches to justify unions, where economic aspects balance labor relationships and political ones emphasize advocacy within the political system.
Step-by-step explanation:
The broad approaches used by institutionalists to justify the existence of unions according to the textbook are economic and political. From the economic perspective, unions contribute to the efficient operation of labor markets and help balance power between employers and workers. Politically, unions represent a collective group within society that engages in the political process to advocate for the interests of its members. Institutionalists saw two broad approaches to justify the existence of unions. These approaches were economic and political.