Too Much Work-in-Process (WIP) results in delays, task switching waste, and an opacity to project status.
Limiting WIP requires discipline from the developers. Like doing a term paper people tend to wait until the last minute to get work done. It is just natural. In Agile we need to fight that natural tendency and push to discipline.
Limiting WIP will result in smoother delivery, what in Lean is called evening out the flow by evening out the flow you will create a more peaceful cycle of development. One without the last minute rush to complete and test (or forget to test).