They allow you to include placeholders for text strings on the template or the schematic sheet that can then be given a value at a later stage. Parameters are an essential feature for annotating your schematic design.
Schematic Capture Improvements in Update 9 Intuitive Parameter Inheritance This update to Altium Designer continues to deliver improvements and enhancements to the software's core technologies, while also addressing many of the ideas raised by customers through the AltiumLive Community's BugCrunch system. Welcome to Update 9 for Altium Designer 21.