There are only about 13-20 full nodes of Steem in the world (At least public nodes) because it is very expensive to run a full node. To rent a server that can run a full node, you need roughly 750 GBs to 1 TBs of RAM. We are talking about some 900 USD per month just for the proper hardware. The RocksDB implementation is a storage layer that takes the data out of the RAM and puts it onto a SSD instead. This means that the cost to run a full node can be significantly reduced. This also means more people can run the steem blockchain which further decentralizes the chain.