All of the answers entered so far are correct but I would like to also add a few thoughts;
Sometimes nothing was left to a descendant to show disapproval. But sometimes the descendant already received their inheritance before the death and nothing was left in the will. In some cases, the child or grandchild was well off, maybe successful in business or married rich, and the benefactor didn't think that they needed any part of the inheritance. And lastly, if the heir moved out of state and had lost contact, the donor might not be sure if they are alive or if they will be available to collect if they cannot be located for some years - and this is important because it might not be possible to close out the probate or the process could be delayed if all of the heirs cannot be located.
If you have read many wills you will often see a comment that the person who the will is written for declares that while they are weak in body, they are sound in mind. This is an important declaration because if it can be proven (or alleged) that the person writing the will was senile, it can be contested. And what is a good cause to bring action that the will writer was senile? They forgot to include a child or a grandchild. Certainly they had senility if they couldn't remember their own family!
The way to show that someone was specifically excluded from the will was to leave them some small amount, usually a dollar. While in most cases this was done to show disfavor, it wasn't always the case. If the person already received their inheritance and they knew they weren't going to get anything in the will, if they were left out of the will, they might be able to challenge it. Money makes people do silly things. After years of experience, lawyers made it standard practice to add a dollar for those who were excluded.
In modern times, some people do this when leaving a tip for the waiter/waitress. If you leave no tip, they might think you forgot. Leaving a penny signals that you were not happy with the service. This is an older custom as most pay by card now and writing $0.00 in the tip box on the credit card bill is a pretty clear indication that you were dissatisfied. I read on a travel site that if you are particularly displeased with a server, you can write a message at the top of the credit card slip because the manager or owner will surely see it.