Partial Update was one of the most wanted features by Cosmos DB customers. In a regular update operation, you need to send the whole JSON document to Cosmos DB. This can be silly if your data model is large and you want to update one field in it. With a regular update, your request object will be large because you need to send the whole data model. Regular Update operation needs more resources from the client/SDK and network bandwidth.
You might think that partial updates might cost fewer request units. Unfortunately, this is not the case. Because Cosmos DB still needs to open the JSON document, change the necessary properties and save the data. Cosmos DB uses almost the same amount of CPU and memory for this operation for a regular update or a partial update.
That it costs just about as much as a full write does reduce the value of partial updates. Still, there is some value in reducing bandwidth requirements or making changes where you don’t know the entire contents of the document up-front.