Being able to concentrate is one of the keys to increasing your productivity. It doesn't matter what you're working on, whether it's a report related to work or a hobby project out in your garage, if you can't focus on the task at hand, it will either not get done or not get done right.
Researchers have been able to show pretty conclusively that an inability to concentrate results in errors and mistakes and generally slow productivity in any environment.
This can take anywhere from a few seconds to a half hour or longer, depending on the complexity of the project and the length of the interruption. This can cause a project to drag out longer than it has to or result in shoddy, inconsistent work.
Sometimes we are working on multiple tasks at a time, such as writing a report in between meetings and phone calls, with the belief that we are being more productive. This is commonly referred to as 'multi-tasking'. However, it's really a series of planned interruptions or distractions and it has the same negative effect on your work as any other sort of distraction.
However, research has shown that if you can set aside blocks of time to concentrate on a task or project, or learn to avoid distractions, you can usually complete it in less time with more consistency and fewer errors.