It's really common, tbh.
As alpaca said, it drives you mental when you are more experienced than the class teacher, and can see them making mistakes, know you could do it better, but have to have the internal wrangle about whether it's better for you to step up and help her improve her teaching for the sake of the kids, or whether it will make it all worse because she will get the hump and think you think you're better than her, etc etc etc.
I'm using female as that's where it started, and because the number of men thinking about going this route would be so vanishingly small as not to bother with.
Loads of women try to do this with young families, as they can see they would have less responsibility in terms of keeping up with paperwork, but will be in a familiar environment with relevant skills.
If you can bear the feeling of not being able to make decisions and are happy to get on and support the class teacher, whilst receiving essentially no monetary benefit, all good.
There are pros and cons. One friend was told she could have the post, but they were only going to pay her as an unqualified candidate, as that was what they were advertising for. She took the job at the base salary and left after a year. Not because of the money, but because she couldn't bear to watch less qualified teachers and not be able to do anything about it.
So, check your control-freaky tendencies, and work out whether you could even cope on a day to day basis knowing the kids were being let down, and being powerless to do anything about it.
(Disclaimer - you might end up with a really good teacher, but in times where NQTs are often preferred as the cheaper option, a second experienced class teacher but paid as a TA would be seen as a good prospect. NQTs are tricky, as often they are brilliant but may need support, but might find having a ten year teacher as their TA quite a terrifying prospect, and feel watched and judged every day...)