You do not want just anyone on your email lists. Email marketing should be targeted to get the best from them and the only way to do this is to know specifics of your subscribers. A long list of personal questions at the sign up stage is a way of putting most off so you need other methods to understand who they are.
You might have decided to try and convert your newsletter subscribers to your direct email marketing list. One method is to include an article of limited general interest. The reader is intrigued and reads on. At the end of the article they are told that one following on or more detailed will be found at the end of a click-through. If you capture them and they subscribe to your email list then you will already know something of their interests.
The same goes for those whom you acquired via staff at a trades fair.