To use SD card reader, you will need to have a computer & a memory card. So the main function of a card reader is to help you work with memory card & a computer. Having connected to PC, you can move, copy, paste or backup your data from your memory card to your computer. Depending on your usage, you can buy a card reader for your needs. Some are hybrid memory card readers that support multiple cards in one place while some card reader only able to support a specific type of memory cards. There are various types of Memory card readers available. You can read data from your memory cards, perform copy, paste, more or backup data from it to your computer with the help of SD card readers. SD card reader is a small USB plug-and-play device that can be used to read data from portable flash memory cards like MicroSD, Mini SD or SD cards. But you can make use of SD card reader to connect your memory card with your computer.
If you have a memory card & you want to connect it with a computer, you can not connect it directly if your PC has no memory card slot.