Sometimes I see recipes posted online that are modified versions of another recipe.
Separately (but which will be related in a moment) my favorite sources for recipes come from Smitten Kitchen*, The Moosewood Restaurant cookbooks, and America's Test Kitchen cookbook(s?).
Neglecting the fact that I've been making mac & cheese from a Test Kitchen recipe without actually following the recipe (only due to laziness, not due to any crazy ideas about being able to improve the recipe), I have a really hard time understanding why you would modify a recipe from any of these three sources. I just feel like... I don't think I'd be able to make a recipe that did that.
Except when a Smitten Kitchen recipe is a modified Test Kitchen recipe. Then I get confused and end up with a lot of really delicious scones in my house. Also, the reason I was looking for scone recipes in the first place was because I found lemon curd at the grocery store and needed a reason to buy it. Now if only I could find clotted cream. Then my tea time dreams would be complete.
*I just went to the page to include the link here. Looks like I'll be making Chocolate Sables in the near future.