It depends if you are looking at skill improvement or validation. For improvements and gaining new skills you can find scenarios of future benefits the company would gain, project you'll be able to accomplish and so on.
For getting certified on technologies you know, you could explain that its very useful to know the industry standards, and how others in the field utilize the tech. Also in some case you need to be officially certified to be able to work or support some technologies. Plus it give the company more certainty that you are keeping up with the technology practices and will be completing the work you are assign the proper way. This is a good bargaining chip if there are standard or regulations that your company needs to meet, such as HIPPA.