When you lease a freelancer you get different bids on different people which are most of the time independent contractors who can't be really easily managed (and you can't what they do exactly.)
Here you get people who can be easily managed andrarely miss deadlines.
You're in control. Plus, they're cheaper than freelancers in most of the cases.
Every day your employee knows more of what your business needs.
Compare this to freelancing where you have to tell your new freelancer what to do over again and what your business is about.
Leasing is usually a long-term investment with long-term successful results.
Your employee will work every day toward improving your business. You get consistent results.
Compare this to any other model in cooperating with people (like freelancing) where you should post different projects all the time in order for your results to become at most very close consistent to the results you get with leasing.
In short you reduce your costs and increase profits. Plus, you work with a reputable company that cooperated with several multi-million businesses.