What does custom installation allow users to do?

Custom installation is designed to give users greater control over the setup process of software applications. Specifically, it allows users to select specific features to install rather than opting for a full installation that includes all possible components. This is particularly useful when a user wants to save disk space or only needs certain functionalities from a software application, ensuring that unnecessary features are not installed. By having the ability to choose what gets installed, users can tailor the software to better meet their needs.

The other options do not capture the essence of what a custom installation offers. Automatic installations are typically pre-defined and don't allow for such specific choices. Deleting existing software is not a function of installation processes but rather a task done via uninstallation. Optimizing system performance might be a result of selectively installing only necessary features, but it is not the primary purpose of a custom installation.