Why does BrickLink append a -1 to so many set numbers?

In the 1980s, there was a major set adjustment to the Lego space sets , in North America. I know, because I have both sets. One is noted as -1 and the other -2, which actually has been helpful in sorting sets. It is arbitrary, and I don't see it as the first or second time the same part number used. But untill Lego group makes an adjustment, or creates an online catloge of all sets, I don't see brick links actions as objectionable.

As jncraton pointed out some numbers have been reused by different sets, however there are other cases too.

A good example is 10242 MINI Cooper. During production of this set TLG has decided to change the box from original squarish size, which is designated as 10242-1 on Bricklink

to rectangular version of the box, so Bricklink used 10242-2 to mark this change.

TLG hasn't always been particularly careful about their set numbering, so the community has taken to adding -1 to the end of the set number to handle cases where there may be duplicate set numbers used by TLG. For example, here's what we refer to as 8858-1:

And 8858-2:

As others have pointed out, this also helps to track minor adjustments to sets (re-releases, box differences, inventory adjustments, etc). Bricklink maintains a list of sets that are similar and have the same official set number but which are assigned unique suffixes for various reasons.