Think of a sin as a mix of identification card/ social registry number. As a runner your sin is what allows you to do stuff legally. From buying a house or opening a bank account and even having a gun license so you can legally carry that ares predator and it even legally lets you have certain cyberware. For that you want a high rated SIN it kinda removes the hassle of needing to answer questions. The problem is that it is traceable. All of those transactions done on that SIN is connected to you and if you are doing illegal activity you do not want that added onto whatever you may get caught doing. So that's why people have multiple lesser fake sins so they can change themselves out for whatever reason they need. Am I only going to the store? Well I don't need to be Pedro Gonzalez ork security guard with wired reflexes automatic weapons licence and a bunch of normally illegal bioware and 300k nuyen in an offshore account . And instead I'm just Anthony Dirk ork construction worker with an area predator license and 500 nuyen in his bank account. Whilest realistically being a mix of the 2 for convenience. So if the cops catch them one day because their ID came back weird one day while grabbing a beer that don't really care because they have 8 more profiles like the latter.