The closure of the West Coast Main Line over January (and now into February) 2016 highlights the need for alternative routes during closures and disruption. Trains have been diverted via the G&SW and East Coast main Lines for the duration, but many passenger services have had to be replaced by buses. The G&SW main line via Dumfries and Kilmarnock is a handy alternative route between Glasgow and Carlisle, but it suffers from a lack of capacity. There are several stretches of single track, and it is not electrified. A double tracked, electrified line would allow the use of Pendolinos, class 350s and electric-hauled freight on the route.
The campaign for Borders Rail has also highlighted the fact that a fully-rebuilt Waverley route to Carlisle would allow Carlisle-Edinburgh trains to be re-routed that way, instead of being replaced by buses. http://www.bbc.co.uk/news/uk-scotland-south-scotland-35412900
Again, the route as built so far is single track and not electrified. In the long term, such upgrades may become necessary.