Suppose you want to use asterisks to mark footnotes.* As soon as you add a second footnote,** you're in trouble because your asterisks disappear and everything between them turns bold.

* I had to escape all of these asterisks.

** I see this happen fairly often to people's comments here.

Asterisk * has other uses too. Even more common than footnotes would be x*y=42 X=42/y

HN automatically finds this example and puts in escapes to make it work.