Most people just think bitters are for fancy craft cocktails or your aunt's Old Fashioned, but I find bitters to be a refreshing and aromatic addition to sparkling water.
Usually, but not always, containing an alcohol base, bitters are basically herbs, roots, or other aromatics in a concentrated, or bittersweet form. Bitters have a long and slightly sordid history. Well before prohibition, they were used medicinally and the British added them to drinks to help cover the taste of quinine which was in tonic water and used to fight off malaria. When prohibition came along, bitters were still available since they were considered "medicinal" and despite their frequently high alcohol content are still sold as alcoholic non-beverage products and taxed as such. Totally weird politics going on here.
The best thing is, you don't need to buy the super expensive fancy bitters; you can make them yourself!