We commonly use USB or Flash Drives to transfer the data or to carry some datas with you while you on work or while travelling. While using USB or Flash drives in any of the operating system avaliable in the market you will see an option to safely remove your device. You can ask a question what is the use of safely removing USB.
When we plug our Flash or USB drives into our system or in a laptop there is some transfer of data from between the devices. Once we finished transferring the data from system to usb or flash devices you may not notice some of the datas are working with some of the application in the background. But when we safely remove the device will help the system to stop those background applications and it won't cause any problems to system and the usb drives.
When just plug off your device from a system or from a laptop it may corrupt the data in the usb drive or the datas copied to that drive. Sometimes this will cause the USB drive with some problems such as it won't get detected on a particular system or i wont allow you to copy data into that device. Hope this information might be useful for you, Happy Computing !