It's actually the port of the webcam. If you look into your logfile, you see what OBS reports about your device. For example this:
22:08:23.952: [DShow Device: 'Video Capture Device'] settings updated:
22:08:23.952: video device: HD Pro Webcam C920
22:08:23.952: video path: \\?\usb#vid_046d&pid_082d&mi_00#6&4e05b28&0&0000#{65e8773d-8f56-11d0-a3b9-00a0c9223196}\global
22:08:23.952: resolution: 640x480
22:08:23.952: fps: 30.00 (interval: 333333)
22:08:23.952: format: YUY2
You see the video path entry. This is the physical name of the device on your Windows machine. By this path, OBS is identifying the webcam. This is different on every machine, it is different for every USB port, and probably (this is something I don't know) different between different cameras of the same type.
So if you unplug your webcam, you have to plug the same device into the same port on the same computer to make it OBS recognize again. Otherwise the path is different and you have to reassign this setting. You don't have to remove and ad re-add the source, you only have to reassign the path in the properties of the source by resetting the "Device" entry.