Shaker bottles are great tools to have if you are a regular drinker of protein or nutritional shakes. They come with either a shaker ball which works like a whisk to blend the powder when you shake it, or with a plastic insert with fine holes which acts in the same way, dispersing the powder so it can be dissolved in the water.
Any cyclist who uses shakes - whether for recovery, before a training session or even for mixing shakes for during rides to take in a water bottle - should have a shaker bottle to make creating the shake as easy as shaking the bottle.
Most shaker bottles have a sports style lid which makes them useful for taking to the gym and for taking on the shake without risking it going everywhere - especially if you're taking it whilst on a stationary bike or treadmill.
They also usually have a measurement scale on the side so you can gauge how much liquid to put in. This is useful for measuring the amount of shake you take on board, especially if you mix the powder with something like milk rather than water.