All of these responses are spot on, in my full time life I work in marketing and one of my responsibilities is market research, I would say that if you are looking to gain investment, or as others have said plan to give up work etc.. then it is essential to have a strong understanding of the market, customer needs and industry insight, not only should this be a key section of your marketing plan within your business plan but this should be something that is undertaken on a regular basis throughout the life time of the business as it is essential to keep in tune with customer insight.
If this is a business that you are planning to start up in your spare time, then market research is still important, however balance it with everything else you have to do.
The key is that market research should help you understand how to market your business and who to, as well as whether the market is bouyant enough to embrace your offering, so however you go about it there is a definate need for it in any business model.