Basically to implement many (fancy+fast) features/ something like coprocessors need to be implemented. Need to be on the server so you lower the amount of data transfer from client/server. Since the db is a foundation, it needs functions locally on the data. And by fast the functions need to be in plain c/c++.
This then can be used for any feature: layers, auto TTL, hyperloglog, everything.
Examples: hbase coprocessors(java), redis scripts(lua), redis modules©, tikv(rust, similar to fdb) coprocessors etc.
Makes sense ?