I can remember having a similar feeling when I passed my skills test at 55 hours - I am licensed to fly anywhere - why do I have to do a checkout to hire at a different aerodrome?
I am now well over the 100 hour mark. I have landed at 26 different airfields so I have no problem with landing away. Having said that, my flying is still far from perfect.
Do I now think that the checkouts and time with instructors is necessary or worthwhile?
Yes and no. If you have passed your PPL - let me stick my neck out and say - yes you do know everything you need to know to fly VFR. Don't throw your training manuals away! They make a lot more interesting reading once you have experience that can relate to them.
You are required to spend an hour with an instructor every two years. Really you don't need more than that if you are not doing a different rating. Having an instructor take you around the circuit a new airfield is useful - on the other hand a briefing would be just as good.
The rest of the time with instructors are due to club rules - which has probably something to do with their insurance. Personally I find I don't learn very much - certainly not much value for money spent. Yes - you will make mistakes and the instructor will point that out to you. You already know what you have done wrong before you are told. They might give you this or that tip - often contradicting what you have been taught and what is in the aircraft's POH. They may also make you feel good by saying "nice approach" etc - so it does pump up your confidence in that sense.
Most of what I have learnt post-PPL is by learning from my own mistakes and reading. Don't forget to read the POH! Good old Flight Simulator is also useful - and it costs you next to nothing. Fly a new route on it, land at a new airfield. Photographic scenery helps. You can also practice PFLs on it - although unless you have a head mouse they are more difficult to do than in real life.