let me answer the first question: why to split a Task, the answer for this is

• Either you want to interrupt some work with an other (By You)
• Or by leveling the resource to maintain the work balance (By the system)

For the second question How??

To split a Task do the following

1. Open the project using Project Professional 2007 (you need to give the user the following permission "Log on to Project Server from Project Professional" from the Global Permission )
2. Open Gantt view, click on Split Task
3. On the bar, click on the place where you want the split to be applied
4. Drag and drop the second portion of the bar to the new location in the chart (new date) 

Best Regards

Bilal Okour