A common piece of advice shared with budding entrepreneurs is that you should start a business with the intention of filling a gap in the market, or providing a service where there is a need. However, a good way to frame this same of advice is to ask: What jobs need doing? and Why is your product or service is worth hiring to do it?
Clayton Christensen uses the example of a fast food chain that wanted to sell more milkshakes - they knew that milkshakes were being purchased, but couldn't increase the number. Their breakthrough came once they understood the job that their customers were hiring a milkshake to do.
See this great video of Clayton telling the story.
The Burger King customers were hiring a milkshake to a) pass the time on their way to work and b) keep them going until 10am. It turned out that milkshakes do the latter job better than the alternatives (apples, doughnuts, toast, and so on), but if these customers arrived at Burger King on their way to work and there was a long queue, then they would just put up with being bored.
So the answer to selling more milkshakes wasn't to make Burger King milkshakes more delicious than McDonalds milkshakes. The answer was to make Burger King milkshakes do the job they were being hired to do, but do it better, by being more readily available to commuters who were in a rush.