Postcards can be a wealth of information. Granted you only have one side of the conversation but quite a bit can be inferred from them, especially if you can put them in chronological series. I agree that you may want to sort them by surname. Postcards can also serve to identify unknown locations found in related photographs. One of the images in my collection was taken in Germany circa 1932 but we didn't know where. That was until I found a post card with an almost identical image on it as that in the photo.
I inherited a large postcard collection, about 20% of which contain correspondence. They are certainly a lesser known resource and there are several methods for preserving them. Early postcards in good condition are valued by collectors. You could consider selling those without letters on them and retaining the others. It depends how they are stored and organized. I know how such a large collection can be overwhelming to manage.