Both devices are used for downloading firmware to the phone, but they represent different stages and support different command sets.
The devices you're seeing are associated with the first two stages. The boot ROM will run, which will run the preloader, and depending on implementation, either show the charging animation or starts the Linux kernel and enters into a mode called kernel power-off charging (KPOC) that shows the charging animation. When you plug in a MediaTek phone, it will turn on, but not fully on.
