When something needs its own budget or its own rules. That's the whole test. Separate campaigns exist to separate: a service with different economics (emergency vs. scheduled work), a geography with its own budget, or brand terms you want reported apart from cold acquisition.
The wrong reason is tidiness: splitting a working campaign fragments conversion data and can restart learning. If ad groups can handle the separation (different themes, same budget rules), stay in one campaign.
A healthy small-business account often ends at two to four campaigns total. If yours is sprouting a campaign per idea, consolidate. Data density beats organizational beauty.