Why did people put 'eth' at the of words?

The suffix "eth" was used in Middle English and Early Modern English to indicate the third person singular present tense of verbs. It is a remnant of Old English verb conjugation, where "eth" or similar forms (like "eth" or "th") signified ongoing action. For example, "he walketh" means "he walks." The usage declined as English evolved, leading to the more modern verb forms we use today.