You've never experienced what goes on in seg if you think it protects the officers, inmates or the institution.
People can develop prison psychosis which drains resources.
Extreme violence. Extreme self-harm. Spitting at and and biting officers or just gnawing through their own flesh to require medical intervention. I have seen this happen.
There are dirty protests where people throw piss and shit on officers, smear it on themselves and all over their cells which someone employed by the estate has to clean up.
And every time there's a serious incident on seg or anywhere else, it drains the resources of the rest of the prison so it can go into lockdown. Visits, including essential legal and medical visits can be cancelled. Inmates who are due to attend court or essential medical appointments will have that cancelled as there aren't enough staff available.
Inmates who are usually allowed out for work or association are not allowed out because the staff available to facilitate and supervise or not available. Inmates may not get meals, medication or even toilet paper.
One of the wings in an English prison i'm familiar with has no in-cell sanitation so no toilet or sink. Inmates have to alert staff to let them out to go to the toilet but when no staff are available they have to piss and shit in cups or whatever else they have available in their cell.