What is the real benefit of tenured positions for universities?
Institutions offer tenure, not (just) because of high-minded abstractions like "academic freedom", but because it makes good business sense.  The benefit of tenure to the institution follows from the benefit to the individual:


From the faculty member's perspective, tenure makes it possible to pursue high-risk/high-impact research ideas without having to worry about having to keep short-term bean-counters happy.  
This makes institutions that offer tenure more attractive to strong researchers because those researchers want an environment that best supports their ability to pursue their research ideas.  
Such researchers, in turn, are very often also the ones who bring in the big grants.  Their high visibility also enhances the reputation of the institution, which attracts more students and alumni donations, etc. etc.


For these reasons, given two otherwise-identical institutions where one offered tenure and the other didn't, the one without tenure would find itself at a significant competitive disadvantage.