The Media Transfer Protocol (MTP) is an extension of the Picture Transfer Protocol (PTP). While PTP was designed for downloading images from digital cameras, MTP was built to handle a wider variety of media files and metadata. When you connect an Android device or a digital media player to a PC, the MTP USB Device driver allows you to browse the device’s internal storage as if it were a portable drive, without giving the computer full control over the file system.
Most modern devices default to "Charging Only" when plugged in for security reasons. mtp usb device