Others have answered the question, but I did want to mention - many people mention that tips are important because employees can be paid below minimum wage, but this varies state by state, and in my experience tipping is standard regardless of this.
Here's a table of minimum wage by state:
https://www.dol.gov/agencies/whd/state/minimum-wage/tipped
The "Minimum Cash Wage" column is the "tipped" minimum wage - this is what employers can pay for jobs that get tips. The "Basic Combined Cash & Tip Minimum Wage Rate" is the minimum wage for non-tipped jobs, as well as the minimum a tipped employee must receive in wage plus tips - if they don't make enough tips, legally the employer must pay the difference.
So for example, in Washington state where I am, the minimum wage is $15.74 whether you're tipped or not, but it's still considered rude not to tip, and 20% is still the standard "good" tip.