1. It can leave you to do what you love best: sell. Lead generation can take up energy and, if it's not successful, it can be discouraging. Letting someone else do the dirty work can leave you with energy to use on the things that further your business: building relationships with clients that lead to sales.
2. It can save you money and time. Creating and mailing out 100 postcards time and time again gets expensive–both for your wallet and your watch. Lead generation companies do large amounts of volume through the post office which gets them discounted rates. These savings are then passed on to you, often making it a cheaper form of marketing than many alternatives.
3. It can make your other efforts worthwhile. Even if you conduct your own search, if you're advertising a seminar for instance, you've still already paid for the food, the location, and the marketing to get people there. If your list doesn't pay off, you're still stuck with the bill. Putting at least part of the work in the hands of professionals can ensure that the effort you've poured into the other aspects of the campaign pays off.