I don't think it's totally necessary. Many folks write specifically with the mindset that it'll go on dev.to but we do try to make the process as easy as possible for folks that do. Out of principle I think many like the idea of publishing to their own platform even if they are also publishing elsewhere.
Also worth noting that dev.to allows you to use a canonical URL so search engines know it's the same post and index it accordingly (good for your SEO).
I have different content here than on my own site; my own site has more specific how-to type stuff so I use dev.to to separate the "culture", "career", etc. type of material.
Also, inspired by an earlier dev.to post which I can't find now, I don't care about SEO and don't have a "hit counter" on my own page, so this is not a worry!
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.