Kingdom Come saves are fragile. The original game had a notorious history of save bloat and script errors. A DLC unlocker forces the game to load quest logic for content you don't officially own. If the unlocker fails to emulate the "Day 1 Patch" or metadata associated with the DLC, your 100-hour save file will throw a "Quest script missing" error, freezing your game during Theresa's exposition dialogue.

This is the boring, safe answer. The official Kingdom Come 2 Season Pass is essentially a legal DLC unlocker. For $40 (estimated), you get all upcoming DLCs (likely 3 major expansions) for 30% less than buying them separately. You also support post-launch bug fixes.