It's because we use different endings depending on formality and politeness. The ending "-ㅂ/습니다" is used when talking in a formal and polite way. for example, 먹다 = makan 먹다 + ㅂ/습니다 = 먹습니다(formal, polite) 먹다 + 어(아/여)요 = 먹어요(informal, polite) 먹다 + 어(아/여) = 먹어(informal, not polite) \^o^/

Formal declarative sentence-final ending (in Korea there are 6 speech levels and for each there is own sentence-final ending) These speech level endings indicate the speaker's interpersonal relationship with the adressers or attitude toward them (social meanings such as intimacy and formality of the situation) 먹습니다 and 먹어 there is used the same verb 먹다(to eat) BUT the difference is that the second example you can use only with your close friends while the first expression is formal.