It's not just a marketing number, its a main indicator of model size and memory usage. Some of what is happening now is trying to see how 'large' the LLMs need to be to function at certain level, for instance it was claimed Llama (65B) had GPT-3 (175B) level performance but at 65B parameters that is a lot less memory usage. It's rough high level indicator of the computational requirements to run the model.