Why do we use "es" for "usted" and "eres" for "tú"?
The word usted is derived from the ancient expression vuestra merced (your mercy), which was used to politely/formally address other people several centuries ago. When you addressed someone as vuestra merced, you were not directly addressing them, but something else ("their mercy"); hence the use of the 3rd person that has been kept to our days.
Many other languages resort to using the plural in their courtesy treatments, such as French (vous) or German (Sie). Even English, though this is masked nowadays. Originally, you was the plural of thou; but it was used so often as a courtesy treatment that the singular form (thou) was deprecated in regular use. This is the reason why, nowadays, English makes no difference between the singular and plural forms of the 2nd person.
"Tú" and "Usted" are the second personal-pronoun. Both of them.
We use "Tú" for friends, siblings, people of our age, sometimes younger people than us, or someone that we have confidence with,
With "Usted" we refer to older people than you, professors, maybe your parents and older familiars, or someone you don't have that much confidence with. It shows some respect for the person.
"Es" and "Eres" are the conjugation of the verb "Ser" ("To be"). We use "Es" with "Usted" and "Eres" with "Tú".