Putting a form on your site to collect information about your visitors is the begining of the most basic and most cost effective marketing techniqe; known as segmentation.
A form allows you, the website owner to gather valuable information from your visitors. the most important piece of information is a way of contacting that visitor again in the future (typically an email address).
Now on your form you should also ask at least 1 question that will seperate your list of visitors into two groups. For instance, if you know which visitors are men and which are women, you can later send out different emails to each group using language that suits each group. An email that is aimed at women is more likely to be enjoyed by a woman than a general email that doesn't take account of her gender.
Alternatively If your website is about holidays, then knowing the age of the visitors might help. There is no point in sending information about summer student vacation hotspots to people who are in retirement.
If you get the message right, then people will let you know their email address and you can begin to build a database of contacts who have agreed to receive emails from you. Remember that under EU law (and the rest of the world will follow soon) you cannot send marketing literature to people unless they have specifically requested it.
Sending Email newsletters can bring visitors back to your site, which can add to your revenues. By offering well written information and incentives you bring visitors back to you site with a simple email linked to your site.
Any big business has a database to market to. Just because you might be a small business or a one-man-band doesn't precvent you from using the tried and trusted marketing techniques that begin with a form on your website.