There are a number of benifits. The most obvious is Performance. It will take less time to backup data across a LAN.
The Agent technology minimizes the transfer of data to the ARCserve host, because the Client Agent sends only the data that is needed for backup, thereby reducing the load on your communications network and accelerating backup. This helps eliminate the series of “request/acknowledge” packets.
It also provides the ability to "Filter" files from the backup and also can increase security.
If you want to use the Disaster Recovery option you will need the client agent in order to create the machine specific disks.