Datomic does a lot more than just avoiding updates/deletes. It splits the database into multiple parts. The transactor doesn't do any data storage. The clients handle all their own query locally. This means that the transactor (the only bottleneck in the system) does not have to handle any queries or storage and can spend 100% of its time handling write transactions. All other components of the system are trivially cacheable and scalable.
The other big advantage is the concept of "the database as a value". As a client, you can easily obtain the database as an immutable value right inside your application. This allows you to do all the queries you want without affecting anyone else.
The other big advantage is the concept of "the database as a value". As a client, you can easily obtain the database as an immutable value right inside your application. This allows you to do all the queries you want without affecting anyone else.