why i use be along with will in some sentences like i will be go for a walk

First your sentence is wrong. Improve it.It will be:- I will be going for a walk. Here, the reason of using "be" is to follow the structure of future continuous tense because it's a sentence of future continuous tense. We also use "be" in this type of sentences:- Ex:- He will be a great scientist. Sita will not be a nurse. In these types of sentences, we use "be" after will to show aim of someone or possibility to become something. See this too:- Ex:- You will be beaten by your brother. This tree will be cut by them. We also use "be" after will while making passive form of simple future sentences.