Why are there two types of future tense used in French and when should each be used?

Similar to the English language, French has two main ways of expressing an event which is going to happen in the future.The first is the standard future tense which can be used to discuss events that will happen or will be happening. For example..."Tomorrow I will eat a sandwich" - "Demain je mangerai un sandwich". The second tense is known as the immediate future tense which describes actions that are more imminent. For example..."I am going to eat a sandwich" - "Je vais manger un sandwich". Each has a different formation (note that the future tense uses the verb as a stem and has an ending added, whereas the immediate future tense uses aller + an infinitive) and the two can be used interchangeably. It is good to show a knowledge of both tense formations in a written answer.