Forms are widely used to acquire different informations about the visitor prior to their conversion, such as name, email or other information website owner deems necessary. Most often it is used to facilitate account creation, subscriptions to newsletters or other push content or providing billing and shipping information when shopping.
While the forms are necessary, they can have extremely detrimental influence on conversions. The practice should be to use the shortest possible forms and require the least possible amount of information keeping it to the bare minimum. Unless you really need the information, do not require it.