The semantics of objects are pretty straightforward, but the assurances that they provide on top of your data is their main value add in this context.

Yes, things like protobufs, or equivalents thereof provide you with a list of auto-generated low-level assurances, but there are often bits of non-trivial business logic about data modification/reading that cannot be easily expressed outside of a object implementation in a Turing-complete language.

They also clearly express a concept of, and when used correctly, boundaries for data ownership, which for non-ephemeral data can be important.