The introduction of the factory made, double and triple pane, insulated glass window has been an important innovation, reducing energy use and improving home comfort. Exploding in popularity in the 1970's and 80's, millions were installed in new homes and as replacement windows in older homes as well.
Unfortunately, most of these manufactured wood windows suffer from details that trap moisture, and are constructed of wood species like white pine, which have no natural resistance to the fungi that rot wood. Integral storm panels (sometimes called "energy panels") trap condensation, and installation mistakes such as improper flashing or misguided caulking, create conditions where moisture leads to decay, especially at corners of sash and sill. If allowed to progress, the damage can become severe.
Still, it is surprising how much can be repaired economically, especially when considering the alternative – special ordering windows and the costs associated with both interior and exterior finishes. Repairs typically are executed from the exterior, so there is less disruption and little or no dust on the interior. Additionally, if epoxy adhesives and decay-resistant wood species are used for the repair, the result will be an assembly that is more durable and rot resistant than a new replacement window.
For a nice short article on the benefits of window restoration see this:
http://www.windowsashbible.com/pages/window-restoration-benefits.php