Having a list will allow you to create a steady mailing list stream of income. Your list is your prospect and customer base and having this provides you with a market that you can offer your products and services to in exchange for payment. In most mailing list cases once you have an established list a good rule of thumb is that every subscriber will produce a minimum of $1 of income per month on average.
Having a list will provide you a tool for performing market research. Your list can be used as a valuable tool to determine if a new product or mailing list service is of interest to others in your niche. It can also be used to solicit information, why not ask your subscribers what they need to be successful and then try to fill that need. Having a list will help establish yourself as an expert in your niche.
In most cases as you build your list and establish a relationship with your subscribers, providing them relevant and useful content on a regular basis you will establish yourself as an expert in your niche. Your subscribers and others that they mailing list influence will begin to see you as someone to go to for information. The above list is only a few of the reasons why you need to build an opt-in mailing list.