Unions even the balance of power between the executives and labour at an organization. Even if WMF conditions are perfect and there's never any abuse of power (which seems unlikely) -- you should want a union in place _before_ an org decides to do layoffs, or force a practice on its workers that they don't want.
I worked at a tech company a few years ago and the standard anti-union cliche from execs was "we already treat you well" but then about 20 months later they laid off 2000 people, including folks on parental leave. A bit late to try and organize a union at that point.
(edit: typo fix)