"By using should in the if-clause we are suggesting that something is unlikely or not particularly probable.
If you should run into Mathews, tell him that he owes me $100. = Your chances of meeting Mathews are rather slim, but if you MEET him, tell him that he owes me some money.
If she should come, ask her to wait. = She is unlikely to come, but if she COMES, ask her to wait. = Should she arrive, ask her to wait.
Should she fail the test, she will lose her job. = If she should fail the test, she will lose her job.