The purpose of going for a soft launch is to test the product acceptance in market or fine tune the product before implementing a fully fledged launch. As opposed to a hard launch not much fanfare is associated with a soft launch.
Before a hard launch of mobile apps in US, many apps are soft launched in smaller countries like Canada where time zone is also similar to US and the app is refined by analysing usage and other data.
Another example could be of software where before a full launch a small launch is done known as beta version, so that defects could be caught and fixed before the final launch
For some new mobile technology company might want quick customer attention and would like to create a buzz before a competitor comes in market with similar product. For such instances it is better to go for a hard launch and gain maximum benefit till the time permits