It doesn't mean they are paid until then... it just says your last day as an employee is that date, giving them time to look for other jobs.
Don't confuse it with being paid to do nothing because that isn't the case.
If you spent a lot of time and money moving to a new country for work then you don't want to do all that again. In the USA, if you aren't a citizen and you have a working TN visa, then you literally have DAYS to find a new job or else you get deported. Thats why its important, because the current laws aren't realistic with finding a new job. They don't take into account the multiple interviews, the slow responses and callbacks. The entire process takes a long time.