SOME 500 million have downloaded Candy Crush, making it the most popular app in the world. As it approaches the astonishing milestone, CHRIS PALK of DabApps considers the importance of a solidly–defined purpose.

CANDY Crush, the inane repetitive game described as 'the Crystal Meth of the commute', has become an addiction for half a billion people.

Its London developer is valued at £4.2 billion and the investment brochure says it has “a repeatable and scalable game development process that is unparalleled in the industry”.

But despite my sweeping statement on how inane it is, it is enjoyable, and has a purpose – to entertain.

So before you start on the app road, some words of wisdom from a person who has designed and built more apps than you can shake a stick at – developing an app with no clear purpose and scalability is a waste of money.

You should be able to define the app's purpose in two sentences or less.

If you can't, it's confusing and not targeted enough to find its own market.

The most successful apps that make a return on investment pretty much fall into at least one of the following two categories of purpose – added value and problem solving, and entertainment. The former make it easier for customers to interact, buy, book and generally do business.

There may be a website that does the same job but the app connects you to what you want faster and without internet access – solving a myriad of problems, particularly when on the move, e.g. TheTrainLine, BA and EasyJet apps.

Then there are apps that give you an added service and are part of your daily life, such as Skype, iPlayer, Spotify. Where would we be without these?

In the latter category, similar to the success of Candy Crush, Minecraft, Jelly Splash and the likes of Angry Birds provide entertainment.

Key to their success is their appeal across the age spectrum globally and their scalability.

The question of whether your app will make you money directly or indirectly is another story.

But if your app hasn't got a clear purpose, it's not going to be valued, used or have a chance to generate a return.