An Ethereum personal node for the Ethereum public network is similar to our Ethereum shared node in that it allows you the functionality of a super easy, no hassle connection to the public network for sending and receiving transactions.
The main difference is an added layer of security in that it is accessible only to you, unlike accessing a shared node along with many others (albeit through your own tunnel).
A full node also is a program that fully validates transactions and blocks. Almost all full nodes also help the network by accepting transactions and blocks from other full nodes, validating those transactions and blocks, and then relaying them to further full nodes.
The slightly higher cost is reflected in it being a single node unique to you, instead of one node providing access to many (which reduces our costs, which we can pass on to you), but some business applications may appreciate or require the extra layer of privacy/security.