What it is. Creatine is best known as a strength supplement, but the brain also uses creatine phosphate as part of its rapid energy-buffering system. That makes the "creatine for the brain" idea biologically plausible, especially when the brain is under energy stress.
What the evidence says. In healthy adults, creatine does not reliably turn everyone into a sharper thinker. The signal is more specific: benefits are most plausible when baseline creatine stores are lower or demand is higher. That includes vegetarians/vegans, older adults, sleep deprivation, mentally demanding tasks and some clinical contexts. Reviews find potential cognitive effects, but the literature is smaller and less consistent than the muscle-performance evidence.
Why vegetarians are interesting. Creatine is found mainly in meat and fish. People eating little or no animal food often have lower muscle creatine stores, and possibly lower dietary creatine exposure overall. That does not mean everyone needs it, but it explains why cognitive trials sometimes see clearer effects in vegetarian participants than in omnivores.
Sleep loss is another clue. The brain uses more energy when it is trying to maintain performance under fatigue. Some studies suggest creatine may help preserve aspects of cognition during sleep deprivation. This does not make it a substitute for sleep, but it may explain why the effect is not obvious on a normal rested day.
How to use it. The standard dose is simple: 3-5 g creatine monohydrate daily. Loading is not necessary for most people. It is inexpensive, well studied and generally safe for healthy kidneys, but people with kidney disease should get medical advice first. The main side effect is a small increase in body weight from water stored with creatine.
Bottom line. Creatine is a proven muscle supplement and a plausible brain-energy support. For cognition, it is most interesting for people with low intake, older adults or periods of sleep/mental stress - not as a universal nootropic.