Some of these contain L-Theanine in significant quantities as well, which is wonderful in combination with caffeine and helps keep you from feeling jittery or anxious especially with high doses of caffeine.
Without caffeine L-Theanine is a mild anxiolytic and is sometimes used for social or general anxiety, but it's not nearly as effective as prescription options. Honestly to my knowledge it's mostly used in combination with caffeine. Some energy drinks include it and I've seen it included in caffeine pills as well.