Creatine phosphate is part of the energy cycle performed in cells. Supplementing with creatine provides a higher capacity for exertion over a short period of time, like getting one or two more reps out of a challenging set.
Supplementing with creatine will result in more glycogen being pulled into and stored within cells. This requires about three times as much water to do. Combined, you’re looking at a few extra ounces of glycogen and the associated water, most of which is being stored within muscle tissue so while it is “water weight” it’s not useless water weight.
It takes a while to build up levels within cells, so you’ll usually do a week of “loading” with higher quantities before settling in to a regular daily intake. General guidelines suggest 10g a day for the first week and then 5g ongoing. Recent studies have indicated the loading phase isn’t necessary, so whether or not you decide to do that is up to you. A 1kg tub lasts damn near forever at that rate, so for the price it’s worth it.
Besides a multivitamin and high protein intake (however you get it), it’s the next highest recommendation I’d have for anyone who trains strength or endurance.