I'll try to answer your question. :) -(스)ㅂ니다 ending appears in a formal polite form of verbs (ex. 이다 → 입니다), while -(으)세요 is a polite ending of imperative sentences and can be translated as "please do XYZ" (also used in greetings and wishes), for example: 주세요 (please, give me ...), 조심하세요 (please, be careful), 울지마세요 (please, don't cry), 힘내세요 (be strong / cheer up) or even 안녕하세요 which literally can be translated as "please be well / healthy / in peace". Compare these two using the same verb as an example: 행복하다 → 행복합니다 = I'm happy / you're happy / someone is happy [in polite formal form] 행복하다 + -(으)세요 → 행복하세요 = (Please) be happy. I hope it will help you a little bit.

Hello, I have been learning Korean for a while and have asked the same question. I said, "Why not just 요 as in 가요, what is the purpose of 세 in the form 가세요?" Actually, 요 and ㅂ니다 are different levels of politeness and formality, as you would know. As another person so well illustrated, you may use 세요 as a common and polite way to ask for things. Generally even with people that you usually use 요 with you would not tell them to 'go!' by saying 가요. Instead, because it is a command, you use 가세요. However, because this is Korean (if you don't know what I mean you will soon enough [emoji]), there is more to it than just making a command polite. Actually, when talking about someone who is esteemed or deserves respect; whether that be the person you are talking to (about them) or someone else (maybe about their mother), you add the particle 시 to the verb. For some reason, 가다->가시다 becomes 가세요 (won't go into that), in the polite form. So, what I'm trying to get at is that while 가요 and 갑니다 give respect TO YOUR AUDIENCE, 가세요 (and the formal respectful version 가십니다) give respect to WHO YOU ARE TALKING ABOUT. Example, 집에 가요. I'm going home. (To someone else) 집에 가세요? Is he/she~Are you going home? In the above examples, you are actually showing the SAME amount of politeness in both examples. Why? Because the 요 means it's in polite form. However, the respect is shown more in the second example for the other person. NOTE: NEVER EVER USE 시/세 ABOUT YOURSELF! You cannot say: 집에 가세요 when someone asks you 'where are you going?'. Instead, use 요 or ㅂ니다 to give politeness/formality to the audience; not respect to yourself. This is a confusing aspect of the Korean language but after a bit you'll get the hang of it. Sometimes I still make mistakes with 시/세. Extra note: you can actually use 시 (or it's variations) with informal speech. E.g 그게 아버님은 주셨어? Did your dad give you that? The 셨 (past 시) shows respect for his audience's father, without giving his audience more respect. 수고하세요