Use this tag when asking for software that needs to run on multiple platforms.
If you're looking for a piece of software that must be supported on / available for multiple platforms, which is important for collaboration issues.
Working together on a project with a group of people, it is not always a choice to force all members to a common platform. So if the same software should be used by all of them (e.g. for compatibility issues, or interoperability), choices are either software that runs on all relevant platforms – or using web-apps.