The share link is a bit more specific as it adds a direct link to the question / answer that operates much like an anchor link (
#) in HTML.
This functionality is much more evident when applied to ANSWERS , because you can directly connect to the relevant section in case there are multiple answers ( not always the accepted or most voted is the most relevant to the specific case, or maybe you want to show the WORST or WRONG answer ). It saves the trouble of searching for the relevant bit, especially on very long pages of elaborated answers ( not really applies in this site , but on the network you can find a lot ).
try opening the share link for my answer and then the link to Rarst´s answer , in separate tabs, and you will immediately see the difference.
you might also notice that the URL is a bit different. there is an added
q for question and
a for answer, and the consequent number is actually incremented by one (1) . for example, the question original ID ( for this very question) is
3803 the first answer (@Rarst) will be
3804 - the next one (@s_ha_dum)
3805 and so on. Mine will probably get
3806 unless someone answers at the same time OR already deleted an answer.
I am not sure of the following, but I will put it anyway : when a question is MOVED or renamed ( not deleted ) the browser URL can fail bringing you to some 404. but the share URL ( which uses only ID ) will not . much like the difference between permalink and ID in wp itself..