@hisham_hm Actually because it can be the source of line changes that are unimportant. Adding or removing whitespace is irrelevant but will show up as a changed line in git. Therefore, removing whitespace early and sticking to keeping it removed keeps linechanges clean too.
@Gargron Whitespace-only diffs are a different/larger issue from trailing-whitespace.
I don't trim trailing whitespace and I still pay attention to avoid unimportant whitespace-only diffs.
Whitespace diffs can happen whether you trim trailing blanks or not (e.g. when people "align" C-style variable declarations then have to change/add one of them).