You were required by an algebra class in high school. Vague equations were brought to life and visualized in your graphs. You helped to make sense of newly taught concepts and fostered a connection between math and the real world. You were a bonafide educational tool.
You were indestructible. This alone is one of your most outstanding features. You took countless drops to tile floors without the slightest hint of damage. I can still recall the clattering sound you make after being pushed off a desk. No other electronic device I had owned has ever taken this much abuse.
You taught me the basics of programming. Crude games written in TI-Basic were open for anyone to tinker with. I would manually enter games line by line from ticalc.org and slowly absorb programming concepts. Modifying existing games quickly led to writing new ones. You were a computer and you taught me program structure, loops, and logic.
You provided hours of entertainment. Boring study halls were spent playing games that could be easily traded and modified with a little ingenuity.
You were useful into later years. I had to replace your memory battery at some point but you did not falter. Today your role is to perform the most basic arithmetic and you do so cheerfully.