Naturally, some engineers look at this and immediately get to work to optimize how data can be transferred over the wire. One definite improvement that can be thought of is to transmit the data via binary rather than text-based format, and this can immediately reduce the amount of data that needs to be loaded onto the network. Technically, this data transmission can be done via RPC (remote procedural calls)— each language probably has a library that will be able to do so by sending binaries in a certain format that the package itself would support. However, this would mean that the support is only specific to that language. However, there could be moments when we need applications of different programming languages to talk to each other. Having an application that uses RPC to communicate only to applications built with the same programming language may prove to be a bad thing in the future. It is best to have something much more generic that can be used across multiple programming languages.
GRPC is one way to do so, and it is just a generic RPC that many languages just happen to have support for, which makes it more popular than RPCs that are only kept within a single programming language.