What difference does it really make?
Actually, a heck of a lot in some cases!
You can relate it to standing with bad body posture when standing up: If you stand with your hip sticking out and your shoulders rolled forward it throws your posture out and eventually you get uncomfortable and need to shift around. Now, put a 'bad posture' into a static position (i.e. you sitting on a bike that hasn't been adjusted to fit you properly) for a couple of hours or more and also add into the equation the forces that you are putting through your body through exertion; the likelyhood of using the wrong muscle groups to push the pedals; and riding on a regular basis there's going to be biomechanical problems and eventually, unnecessary injuries.
Having a bike adjusted to fit you early on is the most important thing you can do. Getting expensive aero wheels or whatever can come later when the engine (that's you) is working properly and can give 100% through the bike.
Hope this helps
I can only concur with the earlier notes: a lot
from my own experience: I always got problems in my neck after 90 minutes on the bike and this became really unbearable at one point. I finally decided to do a real bike fitting and several tweaks were made, the most notable was changing the distance (just changing the value with 10 mm) between the saddle and the aerobars, after that I never got any problems anymore and can cycle for hours (read: Until I run out of energy )
Can only recommend it
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