Although the main distinction between 'will' and 'going to' for the future is usually stated as being for spontaneous decidions and planned decisions respectively, that is a general rule for which there are exceptions galore unless you define the rule with more usages.
'Will' is used to:
State facts about the future
e.g. 'The sun will set at 1725 this evening'
Give predictions about the future which are not based on present evidence
e.g. Liverpool will win the F A Cup (in my opinion as a Liverpool supporter!)
Talk about spontaneous decisions that affect the immediate present or the future
e.g. It's hot in here, I'll open the window
I hear Peter is looking for me. I'll send him a text message later.
The above three are the main but not the only ways in which 'will' can be used. They do however account for a far greater area of usage than the original statement quoted in this thread.
I hope that helps.
- For Teachers