Way to many uses for these types of programs to go into here, but just one example. I type many words that need accents. Sure I could go to Edit-> Special Characters-> Select the Accented vowel I need, or use the keyboard shortcuts to do this, but why? Instead I specify in Textexpander that whenever I type a word that needs an accent it gets automatically replaced.
So I type “cliche” and it is automatically turned into “cliché,” “differance” becomes “différance,” “Zampano” becomes “Zampanó” (a character in House of Leaves). This saves me so much time over the course of a paper, especially all of those with a bunch of foreign words.