If you are walking a new (for you) route, and are walking solo, a good guide is important. It helps you to know what to anticipate to plan the next day's walk, to know what is located slightly off route, and to plan for alternatives, for times when life happens.
If you are in a group, at least one member should have the guide book, accessible to all. All of the above reasons apply IMHO to each member of the group.
If you have walked the route before, then I recommend one of the Wise Pilgrim Guides for your smart phone. They are available in the Apple iTunes store, and I believe are also available through Google for Android devices. They are the best electronic Camino Guides I have yet seen. I believe they are available for most all Camino routes in Spain and Portugal EXCEPT for the Madrid Route.
If you are carrying the phone or tablet anyway for other reasons, these apps add ZERO weight. They also link to your on-board mapping apps. So, they are fully featured. Finally, the fellow who developed these apps pushes regular updates, both off and in-season.
With regards the
Camino Frances, the most popular route, there is even a Wise Pilgrim app for "The Last 151" (km), incase you do not care about the previous 324 km...
I hope this helps.