The network is much less active during evening hours. Backups run more efficiently and are unobtrusive to the clients. Incremental backups run more quickly than full backups and increase data redundancy. Files that do not need to be backed up can be excluded. Hardware compression is built into many tape drives.
Retrospect provides software compression that is slower than hardware.
Backing up all files allows restoration of a user's files, System, applications and working environment if necessary. Not all users may require same frequency of backups. Frequency of backups may change with client's workload. Scripting and Scheduling are easily changed to accommodate these adjustments. Significant performance penalty, but adds security.