Vitess supports MySQL binary protocol. This allows existing applications to connect to Vitess directly without any change, or without using a new driver or connector. This is now the recommended and the most popular protocol for connecting to Vitess.
Features of RPC protocol not supported by SQL protocol
The RPC protocol supports bind variables which allows Vitess to cache query plans providing much better execution times.
The RPC protocols allows you to use event tokens to get the latest binlog position. These can be used for cache invalidation.
Update stream allows you to subscribe to changing rows.
Ability to multiplex multiple request/responses on the same TCP connection.