You should always have a road atlas in your car. Batteries run out and network connections may not be reliable, you will always have access to a paper map if you have one. If you have an atlas you can always navigate yourself out of any problem (diversions, heavy traffic, accidents closing roads etc.) and they are a good sanity check of whatever your GPS or map app is telling you. Better yet, check your route in the atlas before you set out and you will know your route better, and be able to react quicker to any unforeseen problems.