세요 uses the honorific 시, so you will always hear 요 at the end. 세요 can't be used on it's own, as it is conjugated with a verb stem. 주세요 is comes from 주다, which is the verb for give. So, you have the verb stem 주 +시+어요 -> 주세요.

주세요- Please give (it) to me 세요- Is nonsense. It can't be used by itself, it has to have some sort of of verb in front of it. Its a formality, something you attach to a verb to mean "please". For example in english, you can have "running" "reading" "seeing" but you can't have "ing" by itself right? Same thing with "세요".

주세요" seyo===do it please "세요" juseyo==give me please

주 =)