Why do gym people use sipper bottles or shaker bottles instead of normal water bottles?
When compared to a normal water bottle (where you have to unscrew the cap, drink the water and screw the cap tight), sippers allow you to drink water effortlessly (just squeeze the bottle and drink). Sippers are also light weight, durable, air tight, and definitely more attractive. They are extremely useful while cycling, running, etc. Also, it is advised to sip small amounts of water in between your workout. You more often than not tend to gulp more amount of water from a normal water bottle than from a sipper.
Shaker bottles are useful especially for mixing your protein powder. I carry just a shaker bottle to the gym and use it both for drinking water and mixing protein. Why carry two bottles (a shaker and a water bottle) when a shaker serves the purpose of both?