If you think about it, something that is dehydrated, such as a dried fruit, looks almost lifeless and shriveled. On the contrary, when something like, say a tomato, is full of water, it appears plump, full, and appetizing.
Think of your muscles in a similar way! Keeping yourself well-hydrated means greater blood volume and fluid to fill your veins and arteries and amplify the pump effect. You want that, right…?