Every dollar you spend on marketing your conference either takes away from your profits or reduces the amenities offered to your guests. While some will argue that marketing expenses are typically built into the cost of a conference, there is rule that says you have to spend that money. For starters, you can never predict what kind financial challenges you might face as the event draws closer, so it is always better to "extra money" on hand.
The internet by itself offers a wide range of low cost options for marketing events, and yet even here companies will allocate ridiculous amounts of money to gain exposure. In some markets it might be warranted, but when it comes to promoting a conference, there are plenty of PR opportunities to experiment with before writing those big checks.