Why is simple interest useful for planning parts of your financial future?

I believe the answer is: Simple interest is very regular and can be calculated in advance.

This mean that the amount of simple interest would not be affected by the performance in the market, which make it become very predictable and can be calculated in advance so you can make necessary adjustment in your expenditure without compromising your saving.

in this scenario it would be the consumer demand as most influential in making the decision. the band members need to figure out where and from whom they will make the most money, in other words who out of the two consumer groups - football fans or car drivers - will spend their money, who is more likely to contribute to their cause and buy what they are offering.