Does it pay more?
No, you will be more employable in six years, if you are already doing an ACTUAL job
Does it really help your career?
No, it doesn’t, see above. Moreover, it intimidates some employers.
Do you learn more?
Slightly, may be 1 year’s worth. You are unemployable, by the way.
Does it make a significant contribution to humanity?
If your PhD did, you probably can tell. More than 99% PhDs didn’t and are unemployable.
Does it satisfy scientists curiosity?
Hardly. You are just working on a topic your supervisor working on for last 30 years.
Do you enjoy it?
Unlikely. Besides TAing, babysitting students, and following your supervisor orders, the work is mostly repetitive, and it feels at most like going in circles for years. Some may fall in love though, never believe them.
Do you choose your topic?
Very unlikely. You will be working in a field chosen by your supervisor. You will be intimated into it. You can be given some freedom to choose how to do it, as far as you play within the rules.
Should I go if I get a scholarship?
This is like carrot and stick. It is what attracts the bright fouls to do it, like moths to light. Living on a student scholarship salary for 10 years is hardly a living, unless you want to eat ramen and pizza slices most of the time, and look around for every seminar that has free food.
Does it help in networking?
Could be, but your colleagues are your direct competitors for grades, scholarships, and funding, and also attention. It stains many of the friendships developed at grad school. Your supervisor and others have hardly any contacts outside academia.
Do you get a good teaching experience?
If you want to be a teacher go to teacher’s school. Here you will be doing the mundane jobs that permanent faculty hate to do: grading homeworks, doing labs, office hours, and mostly babysitting 1st year students. Being a good TA would not land you a faculty position, research will.