IPFS Desktop is a standalone version of an IPFS node with a user-friendly UI. The application is for users who do not like or do not want to work with a CLI at all.
The garbage collector (part of an IPFS installation) is used to manage the local repository and deletes unpinned content from it, when it is executed or triggered by the values from the config file (normally 10 GB).
The garbage collector is triggered by the values defined in the configuration file, but cannot be started manually by the user. To do this, an appropriate CLI-compatible package (e.g. go-ipfs) must first be installed in order to access the repository. This way is extremely inconvenient and not suitable for a user with little IT knowledge. But IPFS Desktop is especially aimed at these users.
Adding a button, which starts the garbage collector manually, allows users to easily and cleanly remove content that is not pinned (e.g. if illegal content was loaded by mistake). This will attract more users to install a local IPFS node.
One possibility to place this function would be in the settings.