Just as an example, Stihl FS130 grass trimmer can be equipped with Autocut 25-2 cutting head that can hold either 2.4 mm or 2.7 mm cutting line - so within the same trimmer and the same head I can choose which line to use. It looks like the thicker line is stronger so won't wear out that fast and so I should prefer thicker line whenever possible.
Why would I not use the thickest cutting line possible?