Many websites won’t work properly (if at all) if you block all first-party cookies - as these are often used for site navigation and wanted functionality (as here).
By contrast, for Safari, third-party cookies are blocked by default by ITP (although this can modified in Safari’s advanced settings). Third-party cookies are used to facilitate cross-site user-tracking and profiling by other services are blocked - these often being the more insidious side of cookies and web-browsing that many users (perhaps yourself) seek to control.
Don’t forget, recent versions of iOS/iPadOS also include privacy reports:
About App Privacy Report - Apple Support