It's meant to update your glyph set, assuming that you've updated the input string to include new glyphs that were not in the previous set. You can observe the change on mobile by looking at the glyph gallery, which you access by tapping the current glyph icon in the top left of the canvas.

What is the point of the sentence at the bottom then?

I think it doesn’t work because the footer covers it. That seems to be the problem on my iPhone.

Same here