'Suddenly' is not really germane to the use of 'will'; it is the idea of no plan: a simple statement of something the speaker considers sure to happen in the future. 'Going to', meanwhile, indicates an intention, a decision, on the part of the speaker to do something in the future. As you should be able to understand, the decision to use the toilet comes very quickly upon the urge-- and who would want to announce the urge instead of the decision?
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Anonymous"I'm going to read outside.""I will" ("I'll") is often used to show you have reached a conclusion about what course of action to take based on the situation you are currently faced with. It is "spontaneous" in that sense.