Those programs improve the odds of success by offering things like office space, professional services and business advice.
Incubators do not generally have a strict focus on the amount of time a business will spend in the program.
For example, companies at the NYU Poly incubators generally spend 18 months in the program, but other incubators may have a longer time frame.
It eliminates the cost of maintaining a building to house your incubator clients.
However, a physical presence allows the entrepreneurs to assist each other and allows you to monitor their activities, problems and successes.