The DGB is focused on dynamic scaling with the increasing number of resources (i.e. nodes). To accomplish this we use a sharded/parallel chain model. If we use Proof-of-work alone, the hash power would be diluted among the shards making each shard vulnerable to a 51% attack.
If we use Proof-of-stake alone, based solely on signatures of the validating nodes, the vulnerability would be related to loss of theft of the private keys of the individual validators.
The DGB instead starts with PoS for the initial validation, then uses PoW to incorporate processing hashpower with signatures, and then distributed the PoW values among the shards to provide for protection against longterm forks and as a means to deal with potential network partitioning disruptions.