When the business can't open without you.
Many support worker will also come in when sick. This is not office work, it is providing a service that has to be maintained throughout the day. Every worker knows that, if they don't come in, there's a possibility that the business will not be able to replace them and business (and the business's reputation) will suffer, and may explose other workers to customer complaints.
It is hard to take a day off sick. Paid or unpaid sick leave, it really wouldn't make a difference to the work ethic of my job.