Installation testing helps to make sure that application gets installed successfully on different platforms with all other components. Here difference in platform can be in terms of Operating System, device configuration like RAM, CPU speed etc. It is always advisable to first go for beta release and later on when product becomes stable, move to formal release part.
Uninstallation testing helps to verify if the application with all its components gets uninstalled from the system in proper way. A successful uninstallation should make sure that all the folder structures and any generated data during operation should be deleted without any manual effort. After uninstallation, system should be stable and working fine.