Shadow IT is the name given to improvised solutions developed by people more familiar with the business side of things than with IT, and developed using their means. One typical example is the use of spreadsheets.
Of course there are tasks that pop up one single time, in which case a makeshift solution is OK. However, a look at everyday business reality reveals that such stopgaps that were initially needed for a few days or weeks often end up being used for years. In some instances, an actual IT solution exists but would be too complicated to use; this could be because requirements have changed but cannot be accommodated without excessive use of resources, or because these solutions failed to take the users' needs into account in the first place. No matter the cause, the spreadsheet a certain staff member has on his or her C: drive sooner or later becomes a problem if it remains part of essential business processes.
Of course, these non-IT experts come up with such shadow IT solutions in a fraction of the time a "real" IT solution takes to develop. However, a good software solution can deliver invaluable benefits, including:
Shadow IT can prove useful in that it provides an instant solution, but also for the developer to understand the exact requirements. However, sooner or later it is certainly advisable to think about replacing it with a proper IT solution. In order to maximize the benefit to the user, it can be helpful to use technologies that allow a relatively quick development of whatever solution is ultimately decided upon.