What would be the purpose for someone who has the "durable power of attorney" to seek "guardian and conservator" status over the last "incompetent surviving parent?
How would this benefit them regarding the family assets?
P.s I'm not that person, somebody in a family

There is no benefit. There is only the satisfaction of knowing its being done correctly. There may be some pay outs for expenses but that is about it.

They can as guardian, put the person in a nursing home for example against the persons wishes. ( just first thing that come to mind) A POA can only sign for the person, but can not do anything that is against the persons choice

p.s He already has a durable power of attorney.
Because he already made bad decisions towards my dad(incompetent) .The rest of the family doesn't agree with his decisions.
He put our dad(incompetent) in nursing home against his or our wishes. And denying all information to us regarding his health etc...

Combined your posts, please to not start new threads on same subject, merely answer your own question to add more info

First so go and take dad out of the nursing home, is your dad able to live on his own, ( I will be blunt, often adult children not living right there, have no idea of stoves left on, wandering down the sidewalk lost, falling and more)
I had to put my dad in a nursing home some years ago, it was not a popular choice, but no other family member would take him in, and he could not take care of his self, and mom was not able ( although she somehow thought she could)

Next power of attorney does not give him rights to deny your info on medical care, it only lets him sign for your dad, you can talk to your dad and let your dad tell you anything he wants to tell you.

If your dad is incompetent, someone needs to be his guardian, and they need to be close enough to see to his care and needs.

So I guess if you did not want him in a home, what were you going to do ? Take him to live with you ? If so, then go do it, or fight for guardianship in court for the right to see to his care.