The TI-84 Plus Silver Edition is a graphing calculator released in 2004. It includes a USB port, pre-loaded software, APPS, storage, and a removable front and back case.
Probably the backup battery is to preserve the memory and programs while you change the main batteries.
Probablemente la batería de respaldo es para preservar la memoria y los programas mientras cambia las baterías principales. (Gracias a Google Translate.)