Why are there two ways to update documents?

As a CouchDB beginner I'm having a hard time understanding how to update documents.

When I read the docs I find this which is quite confusing for me:

1) Updating an Existing Document

To update an existing document you must specify the current revision number within the _rev parameter.

2) Update Functions

Update handlers are functions that clients can request to invoke server-side logic that will create or update a document.

Source: Chapter 6.1.4 Design Documents

Could you please tell me which way do you prefer to update your documents?

Edit 1:

Let's say the data structure is just a simple car document with some basic fields.

{
    "_id": "123",
    "name": "911",
    "brand": "Porsche",
    "maxHP": "100",
    "owner": "Lorna"
}

Now the owner changes, would you still use option 1? Option 1 has quite a downside, because I can't just edit one field. I need to retrieve every fields first, edit just the owner field and than send back the whole document. I just tried it and I find this quite long-winded. Hmmm...

Solution

Most of the time you want to choose option 1 "Update an existing document"; this operates on a standard document that stores data in the database. The other option relates to design documents, such as views (which are also documents, this is definitely confusing to new CouchDB users), which is something completely different.

Stick with option 1, and good luck :)