I assume that maybe you would want to select multiple cells and change the font/size of the text, but that doesn't appear to be supported by the editor.
It seems that when multiple cells are selected, that the assumed intention is to change the properties of multiple cells at the same time, rather than changing the properties of the text.
You can see the same behaviour in the full editor demo:
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Once you select multiple cells, the main toolbar dims and instead only the contextual toolbar is active.