This is my screencap tool - the 'pin' feature is particularly useful when debugging things. One minor annoyance is it struggles with DPI scaling across screens (i.e. multi-screen with different scaling factors on different screens). There's a long thread on github with workarounds.
What does pinning do? The docs just say "--pin: Pin the screenshot"
It's like a freeze frame for the section of the screen you snip. But you can move it around to other places.