In my view, the real benefit of tenure is to create a class of faculty members who identify closely with an institution, who feel the department and the university is theirs. The tenured faculty do not own the university in any literal sense, but they are like partners in a law firm in a specific sense: they feel the university is, essentially, the faculty of that university, not a set of buildings. I know that I feel that way, as a tenured faculty member. If you think of the collective expertise of a university faculty amounts to, you will see that is is one of the most impressive phenomena of human culture. This is true even of a mid-level university such as the one I teach at. Of course, those without tenure on the tenure track still want to be promoted, so they also aspire to this sense of identification if they don't feel it yet.
Job security creates longer terms of employment and a sense of identification. It also means people move less often, sometimes creating frustration (for the individual) and stagnation (for the institution). Once you have tenure it is very difficult to move, because departments only want to hire Assistant Professors, administrators, or stars. Although I moved once after tenure, I'm not sure I'll ever be able to again. I pay a heavy price, then, for the security I receive.
To create this feeling of identification, you need a sense of autonomy. This is what we normally call "academic freedom," or the idea that a senior faculty member can be trusted to choose his / her own intellectual path. I could write a book about jazz if I wanted to. I probably won't, but I could, and this feeling of autonomy helps me because even working in a narrow sub-field (as I do). I know my decisions won't be questioned by administrators who have no clue about what a valid intellectual project looks like. Longer terms of employment also take into account the up and downs of scholarship. I could have a relatively less impressive five year span and I think I should be allowed that, frankly.
Administrators come and go. I've seen seven deans in 15 years. The faculty are a stabilizing force counterbalancing whatever the new trends in Higher Education might be.
People hired to teach courses on the adjunct track, payed by the individual course, typically feel little sense of deep identification with the university. The move to abolish tenure aims to make everyone, more or less, an adjunct. Even in fields where tt positions are hard to come by, the existence of some jobs like that offers a powerful incentive. If most or all jobs become contingent, then it is hard to imagine anyone going into academia any more in the first place. Even highschool teachers have tenure, after all.