Computers don't care about your maps and will plot a great circle route just fine. However, in the event of system failure you want a real backup. Which is why the navy still teaches people how to use a sextant. http://www.npr.org/2016/02/22/467210492/u-s-navy-brings-back...

Note: The actual deviation from a great circle route is generally fairly minimal in practice.

PS: GPS is just another system that really can fail.