How can I decide if specific business function should be hub site collection or not?

tom_s1865 Check below Microsoft official documentations, it will give you idea when to use SharePoint hub sites:

1. What is a SharePoint hub site? 
2. Planning your SharePoint hub sites 

From 1st link:

SharePoint hub sites help you meet the needs of your organization by connecting and organizing sites based on project, department, division, region, etc. making it easier to:

• Discover related content such as news and other site activities

• Apply common navigation, branding, and site structure across associated sites

• Search across all associated sites

If you have above requirements for your business function, you should use hub site collection.

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tom_s1865 if you want all the associated sites to have the same color scheme as the top-level hub site or if you want to display news from each associated site on the hub site then make it a hub site.

Rob
Los Gallardos
Intranet, SharePoint and Power Platform Manager (and classic 1967 Morris Traveller driver)