These templates allow a printer to quickly line up a film positive with a screen to ensure proper registration. Shops can order these items pre-made from manufacturers, and more experienced printers can build their own over time.
When it comes to pre-production, templates and jigs are valuable tools of the trade that allow printers to register screens faster and start printing. This is especially true for multicolor jobs when printers don't have the time to worry about registration inaccuracies.
Source: Caleb Morgan, Texsource Screenprinting Supply