Novelist Ben Dolnick on the major forms of asides in prose:
A clause set off by em dashes is like dropping underwater while swimming breaststroke — just a quick dip before popping back to the sentence’s surface. A parenthetical clause is more like diving down to the pool bottom to pick up a coin. And a footnote is a full-blown scuba dive — you have strapped on equipment and left the surface behind and you had better, after going to all that trouble, see something interesting down there.
I love that description of footnotes. Just so.
For me, non-fiction prose has always been all about wrangling the profusion of tangents and digressions with style, grammar, punctuation, typography, and design. My use footnotes in particular has become a major feature of my writing and a trademark of PainScience.com.