As a rule, the default settings provided by Veeam are designed to provide a good balance between backup size and backup and restore performance and resource usage during the backup process. However, given an abundance of resources or other specifics of the environment, it might be useful to change the defaults in particular circumstances. For example, if you know that CPU resources are plentiful, and backups are unable to make full use of the CPU due to other bottlenecks (disk/network), it might be worth increasing the compression level.