VisualEditor, the visual interface to edit wiki pages, includes a tool to easily edit templates. However, in order for that tool to be most useful, it needs to be able to pull information about the template; mostly, it needs a list of possible parameters for the template, and their characteristics.
Without TemplateData, each parameter needs to be added manually when inserting a new template.
No information is provided about what the value of the parameter should be.
With TemplateData, a list of supported parameters (and their description) is provided for the template.
TemplateData provides context for each parameter, and makes it much easier for users to modify templates.
One of the things we need volunteers to do is the adding of TemplateData to templates (particularly high-profile ones, such as infoboxes) so that they are easy to edit.
The TemplateData help page explains how to add TemplateData information to a template.
You can probably think of some pretty important templates on your wiki very easily; experienced users know pretty well which templates are a big deal, and will inevitably find out when they try to use them and discover there isn't any TemplateData.
You can find an automatically-generated list of most used templates on your wiki on the Special:MostUsedTemplates special page.