blus that get released a second time around???

[avncars]

i dont know how to word this question, but i shall try my best...i know alot of movies that come out on bluray get rerealeased some time later. does that mean that the second time it comes out it has better picture/audio quality, or extras or something? if so, does that mean that sometime down the road there are going to be much better versions of the blus i own now? and if there isnt a change in quality, why do blurays already released come out again? and the last thing is, why every once in a while when i put a bluray in, before it starts it says (updates are available for this disc, do you want to continue now or at a later date?) what kind of updates are they talking about, cause nothing seems to change when i do update a disc...

not sure this is going to make sense to anybody, but the thought of having to rebuy all the movies i already have in bluray form inorder to have better picture/audio quality really sucks! if anyone can explain any of this, specially why they get rereleased would be awesome...