Greek letters are used because:

- **Tradition:** they have always been used
- **Symbolic representation:** they are compact and pretty
- **Clarity and distinctiveness:** each symbol must be unique and there are only so many letters
- **Standardization:** everyone knows Greek, so it's much better than everyone using their own notation
- **Flexibility:** there are a lot of symbols, and sometimes the symbol's design can help convey what it's trying to represent, e.g. an arrow-shaped symbol representing an arrow
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