With software encryption, unencrypted data causes bottlenecks when being processed into secure data. This increases significantly the load on the processor and reduces the potential throughput of the encrypted text.
Using hardware encryption, unencrypted text can be converted to secure data in real-time without bottlenecking system resources.
One example of this "bottleneck" is encryption/decryption for VPN traffic, which some implementations use the VIA chip by the way. A router, firewall or VPN concentrator must continually encrypt and decrypt network traffic traveling through the tunnel.