Many people make the mistake of thinking this just means after launch and only to your customers. However, if you aren’t constantly marketing internally you might not get the support and effort across your company that you will ultimately need.
For product managers you have to get a huge number of groups on your side and excited about what you are doing, including sales, marketing, executives, operations, support and others. Since you have no direct authority over them you will have to rely on being able to influence them. As such it is critical that they understand what your product is all about, why it is important to your company and how it will affect them and allow them to succeed.
For example, you should market to your salespeople so that they understand why they should sell your product, how they can EASILY make money doing so, what tools you have made available to them to make it easy and what the best approach is. You should also be marketing to your sales executives to ensure the sales compensation plan doesn’t penalize your product – salespeople take the path of least resistance to products that make them a lot of money while meeting customer needs. If your product isn’t worth their time then it doesn’t matter how good it is.