Why do Japanese people say desu?

Answer 1

The word です (desu) is used extensively in Japanese as it means 'is/are/be' in Japanese. So Japanese people use です when they want to say that something 'is' something else.です is often thought to be a contraction of であります (dearimasu), which is less common in modern Japanese.

Example: コンピューターは便利です (kompyuutaa wa benri desu) - "Computers are convenient."