At most or all table service restaurants servers have to tip out other staff members like bussers,bartenders or food runners of their tips.They usually tip them based on sales so for example if that server has a table that spends $100 they have to tip those other employees around $5 or 5% in total ,so if you spend $100 and don't tip that server is basically paying $5 out of his pocket to wait on you.That's why tipping at least 15%-20% for good service is a must.