While I am personally a proponent of using multiple .5 liter reusable water bottles attached to the front of my rucksack, to shift some weight to the front, there is another important point that has not been mentioned.
Across the
Camino Frances, there are many fuentes located in rural areas. Many of these areas are also agricultural areas. Some of the farming areas have shallow springs or wells supplying them that have been contaminated by overuse of pesticides and fertilizer.
The point of this, and a good rule of thumb, is that tap water in towns can be presumed potable, like in cafes, albergues, and hostels. On other stretches, pilgrims are advised to carry extra water, as the fountains / fuentes are likely contaminated.
Over the past several years, most of the reported cases of gastrointestinal illness could be traced to using "sketchy" Fuentes / fountains. In particular, the segment from Carrion de los Condes to Sahagun is such a "dry" stretch. However, towns like Moratinos enjoy safe, potable "city water" supplies.
I hope this helps.