Attaching a second monitor doesn't suddenly make your main display useless.
One of the things I completely fail to understand is why most people I work with close their laptop displays when docked to secondary monitors.
I have a 34" 3440x1440 ultrawide monitor at home on my desk, and I use that plus my Macbook's display.
At work, I use 24x 24" 1440p monitors, and I use thoes plus my Surface Pro's 12" display.
When I'm out and about, on my Macbook or on my Surface, I definitely miss the extended displays. A lot.