What is the advantage of installing a desktop client for 1PW over just using "1Password" extension?

You can also manage items en masse in the desktop app - for example, copying a bunch of items from one vault to another.

(For anyone who is interested, you can select multiple items in 1Password for Windows by using ctrl + click to select multiple individual items, or ctrl + a to select all items in a list. Very useful if you have a bunch of items in one vault that you decide you want to delete, archive, or move to another vault without having to click, move, click, move for each one!)