There’s a number of purposes and it depends which modules you’re using, so here’s a couple of the highlights

If you’re using time cards

• Work Calendars are a required field on the Employee’s screen.
• Work Calendars determine the default start time for time card entries
• Work Calendars also determine the default hours entered for time card normalization and when the “preload holidays” button is used

If you’re using Payroll

• Work Calendars are required for the set-up of the Payroll module:  https://help.acumatica.com/(W(14))/Help?ScreenId=ShowWiki&pageid=227c451b-f4e0-48bf-aabb-39d6c759f63b

If you’re using Sales Orders or Purchase Orders

• Work Calendars can be linked to the Ship Via codes to determine delivery/receipt date for orders

If you’re using Manufacturing

• Work Calendars can be used for scheduling production at the Work Centers.
  https://help.acumatica.com/(W(13))/Help?ScreenId=ShowWiki&pageid=b84668ca-4cbb-4ebc-b627-bc76489d6ad7

There’s other places that they’re used, but those are the ones on the top of my head.