Newsletters are all about client retention. By communicating the value that you bring to the relationship between yourself and your client, you are less likely to lose them when rates go up or procedures change.
If you do not stay in contact, your client can be lured away or think that they can do a better job.
Make sure they know why you are the preferred vendor in your market on a regular basis. Not only does this show that you are the authority in their market, it also keeps the lines of communication open. If they have questions, they will be more likely to ask you than someone else.