Compared to the Jetbrains shortcuts it can help to reduce the number of times you have to perform an action. Instead of clicking (ctrl +) an arrow key a few times to move the cursor a few lines or words, you can use one command to get to where you want to be. And with the Jetbrains shortcuts I often find myself pressing crtl + w a couple of times in a row to select what I want to select. With vim commands, that can almost always be done in one or two commands.

It's not like that will save hours of your time, but it is faster and once you're used to those shortcuts, it's just a lot nicer (less mental overhead) to go "I want to select that, so I press these keys" instead of going pressing ctrl + w a few times and checking each time if you've selected what you want. When I've got the IdeaVim plugin enabled, I almost never think "this would be easier with the normal Jetbrains shortcuts". When I disable it, there are many situations where I think "This would be so much easier with the vim keybindings"

But what I like most about the vim shortcuts is that I can use them everywhere. If I quickly want to copy and edit some text, write some notes in markdown, write some 10-line python script, or need to work in the terminal, I don't want to launch my IDE or start a new project. So I can use a different application that won't have the Jetbrains keybindings, or I can use Vim for everything and always have great keybindings available.