It sounds like tbey were effectively asking your permission as close kin, to take this invasive step at sustaining her life a little longer.
I don't think that was the reason. The nurse administrator had already tried to put the catheter in several times, because by this point, my sister was non-ambulatory, and was wetting the bed. They wanted to insert the catheter to keep her from laying in urine, and them having to regularly move her about in the bed to change the sheets. Without being drugged enough, she wouldn't lie still for them to put the catheter in. And since they were already giving her something to relax her, and keep her comfortable, it only made sense to me, to give her enough to relax her even more so she wouldn't even know what was going on. So when the nurse administrator asked me what I thought would help get the job done, I told them to knock her out, which they did, and the catheter went in easily.