Flexibility in choosing when and where you work, plus the ability to develop your own skills and take responsibility for your own development (not many companies put much effort in these days). I would imagine others have their own reasons.Running your affairs as a “company” with capex and claiming vat back on cars or whatever rather than just paying tax like everyone else.
Allow me to turn it around. If there are no breaks available why do it? You can move jobs as many times as you like without being a contractor. 10 gigs in 5 years looks dodgy as hell on a CV regardless of the tax arrangements.
IR35 isn't a tax break, it's a set of "rules" to determine if someone is a disguised employee rather than a legitimate contractor.