1) Efficiency - If you, like most people, buy a reusable glass cup and keep it for years, you will likely end up washing it over 300 times. Consider how many paper cups you would have to purchase to replace the usage of only one reusable cup. A glass cup could last for 100 years, while a plastic cup can last over 10. A disposable cup is not made to be washed by hand or in the dishwasher because chemicals in the plastic can break down and lead to illness.
2) Beauty - Imagine sitting at a nice dining table under dim candlelight, only instead of using elegant glassware your host uses red solo cups. This would kind of dampen the mood, does it not? Reusable cups tend to be better-looking. Even cheaper cups made out of plastic are usually better-colored and better-designed than are disposable cups.
3) Environmentally friendliness - Assuming that the reusable cup you purchase will be used 300 or more times, it is going to create a much smaller environmental impact than 300 or more disposable cups will. Plastic and styrofoam cups will sit in landfills for up to hundreds of years or be burned and released into the atmosphere. Paper cups require a lot of energy to produce and contribute to the depletion of forests around the world. A reusable cup requires very little effort and energy to wash, especially if you simply wash it by hand. Using reusable cups is an excellent way to safeguard the world for future generations.