There is a specified ratio of water to cement marked on the bag, but it’s a guideline.
Sometimes you want to adjust the final properties of cement, and water ratios can help you do that.
For extra strength, use less water.
Less water in the mix also makes the final cured product absorb less water.
So if you’re mixing cement to use around water, or areas prone to water, you may want to use less water in your mix.
It’s in the textbook, but we experimented in Algonquin College’s masonry lab, making a wide range of cements with differing ratios of water and then crushing them after they cured. Significant differences appeared, at a range of 50% stength variations from wettest to driest.