Be going to vs, will

(a) She is going to succeed because she works hard
(b) She will succeed because she works hard
Be going to and will mean the same when they are used to make prediction about the future. (a) and (b) have the same meaning.
(c) I bought some woods because I am going to build a bookcase for my apartment.
Be going to (but not will) is used to express a prior plan (i.e., a plan made before the moment of speaking). In (c): the speaker plans to build a bookcase
(d) This car is too heavy for you to carry alone. I’ll help you.
Will (but not be going to) is used to express a decision the speaker makes at the moment of speaking. In (d): The speaker decides to help at the immediate present moment; he didn’t have a prior plan or intention to help.