These garden sprayers are also primed by hand with a piston.
High pressure garden sprayers have the advantage of being mounted on your back, making them easier to use over larger areas and are useful for spraying around and up high, e.g. into trees, since the operator doesn't have to stop and start, or bend and lift the tank each time.
The tanks are usually constructed from polypropylene, UV resistant to avoid damage from the sun, lightweight, strong and corrosion proof.
They usually come with shoulder straps or double shoulder harnesses as standard. They start with capacities of around 15 litres, up to a top end of around 20 litres. (For example see image below)