Why do people install roll bars in 914s when they already have one? Major body rust?

Might be required (race car), a good idea (track day/DE car), for "period correctness" (a replica/tribute build) or, like me, for improving chassis stiffness to handle a high(er) power engine.
I'd never consider adding chassis stiffening to counteract rust. Repair the rust properly.
Then determine what you want to do with the car. THEN add (or not) roll cage or other chassis stiffening components.
But that's just me.

As tyaboy said, performance and safety, not a rust correction. Mark
Click to view attachment

As you stated, it already has a roll bar built in. Unless you race, there's no reason to add one to your daily driver. I think most would argue that it's "less safe", unless you're committed to wearing a helmet on the way to work. It's just one more extremely hard object to bounce your brains off of. Just my 2 cents.

Perhaps a padded roll bar is the best way to go. Anyway, the interior is crowded enough as it is.

they make a roll bar that has the back hoop and then comes down along side of seats and mounts on forward longs. Safe and adds rigidity. Mark

I'm not sure padding would help much. It is not going the spread the impact across a larger area like a helmet will.

I agree
Roll bar padding was designed in conjunction with a helmet
Padding alone is not enough

Given all the above commentary, then why install an ugly tubular roll bar when the car already comes with one & you don't race? I know, it looks cool.

This is one style I found.Click to view attachment

I disagree about the safety claim for a street car. Padded toolbars and cages are designed to provide crash protection when a helmet is worn, and actually would increase the likelihood of injury if your bare head could come in contact with the bar.

Unless they‘re required by your sanctioning body, don‘t bother. If desired, you can stiffen up the longitudinal panels to help a bit, but probably overkill forma street car.