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.
I think you might be misunderstanding the purpose of tenure. While your view from the teacher's side might be correct (wanting to avoid worrying about contract renewal) the purpose for a school to offer tenure is supposed to be putting an educator in a position where he/she can teach whatever they feel is best without worrying about being fired if the school doesn't think it politically appropriate. For example, if someone was teaching about communism during the 1960's in the US, the school might want to fire that teacher. Tenure shows that the school believes in scholarship over politics.
As a side note, recent studies show that this is not at all what happens. Students learn more from non-tenured teachers than tenured ones.
As to your question, the responsibilities are the same as any other professional position - do your job. You certainly can leave if you find better opportunities - it's not a prison, it's a job.
I think there are some odd accidental assumptions in the question. For the U.S. system, although there is substantial drift in the last 10 years, the idea was not only that people should teach what their best judgement indicated, without worry of censure or loss-of-job, but that also their research/scholarship should reflect best-understanding rather than politics... especially given the transience and partisanship of politics.
There is also the idea that in otherwise-profitable enterprises people might not want to put any effort into teaching at all, thus not want to participate in a "university" (with students), without an otherwise-extraordinary promise of more-or-less-endless job security. Some smart, able people, not terribly interested in money, beyond a certain point, can be ensnared by the "care-free" aspect of a tenured faculty position.
"Even" in the U.S., in recent years there has been a push to "contract" faculty positions, indeed. In happy times, these seem to be no worse than tenured positions. However, obviously, in the next economic downturn the administration will have the easy option of terminating as many contract employees as seems convenient.
Yes, this is part of the increased corporatization of U.S. (and other) colleges and universities. Of course, we should understand that we are at the end of a sort of "golden age" between the pre-WWI times that only the upper-classes' children "went to college", apart from seminary students, and after the post-WWII time where the "GI Bill" financed returning veterans' college educations to avoid flooding the job market... which was already in disarray after all the women who'd been "allowed" to work in factory jobs and such in wartime were expected (or forced) to quit and "go home"... so there was an artificial surge both in the numbers of college enrollments and in the socio-economic goals.
And more complications currently...