Privacy through uniformity, operational security by routine, herd immunity for privacy, traffic normalization, "anonymity set expansion", "nothing to hide" paradox, etc.

I.e., if you use Tor for "normie sites", then the fact that someone can be seen using Tor is no longer a reliable proxy for detecting them trying to see/do something confidential and it becomes harder to identify & target journalists, etc. just because they're using Tor.

Tor Browser has ~1M daily users. Tons of people use it for hitting sites that may be blocked in their country or they want to have some privacy like view pregnancy or health related articles and etc.

In addition to the reasons in sibling comment, this also acts as a filter for low-quality ad-based sites; same reason I close just about any website that gives me a popup about a ToS agreement.