Even if you aren't experiencing glitches, installing 2.0.6 is crucial for one reason:
Some players noticed that the inventories of visiting NPCs like Saharah, Gulliver, and Leif became fixed after the update. Previously, time travelers could jump between two days to reset and farm these shops, but Version 2.0.6 appears to have locked these inventories to prevent that exploit.
Stay tuned to official Nintendo support channels and trusted homebrew forums for any follow-up patches. As of this writing, 2.0.6 appears to be the terminal update for Animal Crossing: New Horizons.
Nintendo reportedly learned of these bugs through a bug bounty program and issued the patch before any widespread public exploitation occurred. Gameplay Adjustments and "Stability"
The most plausible theories: