Conversion is where the edge cases live
CSV looks trivial until a field contains a comma, a quotation mark or a line break, at which point a naive split produces a file that is subtly wrong rather than obviously broken. The same is true of YAML, where an unquoted value can change type on you and a country code like NO becomes the boolean false.
These convert with the rules of each format actually applied, which mostly matters on the day your data contains the one row that breaks the shortcut you would otherwise have written yourself.
Nothing is uploaded
A CSV export is one of the most sensitive files a person casually pastes into a web tool - it is frequently a customer list, an order history or an internal report. Converting it on a server means it existed on that server, whatever the policy says afterwards.
These parse the file in your browser. There is no upload step, no size limit imposed by someone else, and no copy left behind.