Incubators can provide a useful range of services for a start-up company. Many incubators will have staff members who can help with your business plan or will have access to experts and mentors who volunteer their time to help.
From an investor's perspective, working with an incubator can lend you some credibility. Engaging in an incubator's service demonstrates your commitment. Working with a reputable incubator can also give a potential investor some idea as to the type of resources and assistance you have received. This may increase their confidence and willingness to invest.
You can find more information on incubators as well as find one in your area through the National Business Incubator Association at http://www.nbia.org/