What is stability in sorting algorithms and why is it important?

Question

I'm very curious, why stability is or is not important in sorting algorithms?

Answer 1

the records are sorted with respect to one primary key but not all primary keys;

two records with the same primary key appear in the same order after sorting as they did before sorting

Ex.

Johnson, David

Johnson,Steve

vs.

Johnson,Steve

Johnson,David

They are technically sorted by last name, but not stable.