1. When selecting two or more section blocks, e.g., patterns, the inspector controls change from content only to multi-block controls with styles and so on.
2. When selecting mixed multiple blocks that include sections/patterns you can still transform the blocks in the toolbar.

See #71512 (comment) for context.

Why disable?

Content only mode describes an editing mode in which the inner block tree of section blocks are filtered to only display common content blocks, and with limited editing in the toolbar and inspector sidebar.

The different toolbar and inspector states of section blocks make them incompatible with regular blocks.

The idea is to make things more consistent with other "incompatible" multi-block combinations - in these cases n blocks is displayed only in the sidebar.