The tipping business is too often replacing peoples wages and its not right.
That's why some restaurants include the tip on the bill when it comes to large groups, especially if they are celebrating an event of some kind.
Wait staff often get paid less than minimum wage, and their pay is determined by how many tables they cover over their shift. Waiting on a larger group, obviously takes more time and skill. Should it be an event of any kind, the group will often stay longer and enjoy dessert, coffee or chat. If the wait staff is required to cover that, and it is cutting into the number of tables they can cover, they will get a decent tip to cover what they would be missing otherwise.
Now, the tip can mean quite a lot of money on a large table at a nice restaurant. If you have an 8 top with drinks (especially alcohol), bottle of wine, appetizers, food and dessert, and it runs a $650 bill, then at the required 15% added for a tip, that's a $97.50 tip. If you have two folks leave a $20 on the table thinking that has covered it (at a restaurant where it is not included in the bill), then they are $57.50 short of the mark, and that will make a significant difference in the wait staff's pay.
Plus, in most places, the wait staff have to 'tip out' the bartender, bus boys, hostess/host, and in some cases the kitchen. If they don't, then the service they get from those folks will suffer.
Also, some states have adopted a means by which to collect taxes on tips not based on what the wait staff is actually tipped. Where taxes are generally not determined on a full 15%, it can be anywhere from 10-13% of the wait staff's 'bank' (the total of what they sold during their shift). The wait staff can be required to pay the taxes on that amount no matter what they received in tips.
Credit card receipts also include mandatory taxes, because there is a record of the tip. If you choose to 'overtip' and you are paying with a credit card, tip the extra in cash. Where that seems like it could be cheating, as a whole, wait staff generally fall short of the required 13% total they will be taxed on regarding their 'bank' in some states. "It all comes out in the wash" as they say, and what the wait staff 'tips out' to other personnel is not recorded in any form.