Say a company hires you to take some images for their website. In the negotiations back and forth the parties neglect to include an assignment or license for the images created.
You decide to get cheeky and sue for copyright infringement. After all, they don’t have a license to the work or own it according to the terms of the contract they signed.
If this sounds cheeky, it is. If you think a court would not find it amusing, you are right.
The court would likely create an implied license: saying that your behavior implied that you were granting a license to the company that hired you. It’s always better to have the terms of the license spelled out. When a court creates an implied license, you never know the full extent of the license. By negotiating the terms between the parties, you eliminate doubt about the parties’ rights.