Some laptops include a VGA output (nowadays some have HDMI to go into a TV instead).
VGA port has 3 rows of 5 holes, for a video lead to go into a monitor or LCD screen. Although called a VGA port, it may well handle higher resolution (SVGA) displays too.
Some laptops include this to allow for a bigger 'desktop' area (you can pull a browser window from the laptop screen onto the second (plugged in) screen.
Others allow the display to be (a) on the laptop screen only, (b) on external connection, and not shown on the laptop screen, or (c) shown on both. It's therefore possible to login using the laptop's display without showing other people, and then switch to share whatever is on screen with other viewers.
The VGA port would allow any combination of other devices to be plugged in from a single LCD diaplay so you could have say 6 or 8 people seeing the screens, to using a projector so a large audience could view, or via a VGA splitter, to feed anything from 2 extra screens to a dozen or more.
For the average desktop PC, you often have a standard display port on the motherboard, but for some applications people can spend 100 to 200 pounds for a different video card, allowing better quality or larger images to be displayed.
I've never heard of trying to use an extra video card with a laptop, but I guess if you have seen mention, it is the difference between having, or not having, an extra port to display images on a separate monitor.
If I've not answered the "why would a laptop need one" (eg VGA port for showing a presentation to a larger audience, mentioned above) then perhaps you can point to what laptops you are comparing, please?