Imagine software as a snow ball that you want to move forward. As uncle Bob said, if you don't put aside time to clean software, in time its inertia will increase. In the beginning the ball will be light and you will add features, i.e. move the ball easily. With time the code will become messier and changes that took 1 day in the beginning start to take first a couple of days and later weeks. The snow ball gets heavier whenever you move it further. In other engineering disciplines, the more you work on a product, the better it gets, at least it doesn't get much worse. Good luck to software project managers who try to estimate when the project will be done, because the further down the road, the less reliable the estimates are. For any non-trivial project, you can only come up with reasonable time estimates if the code is continuously cleaned up.
