The seated shoulder press focuses on the Deltoids (posterior, medial, and anterior shoulder) as well as the triceps, traps & upper chest. The deltoid muscles are responsible for the shape and size of your shoulders. So using this machine can lead to increased muscle mass in your shoulders.
Other benefits that can can come from using a shoulder press include improved strengt and stability which is important for everyday activities that require lifting/carrying. As well as the range of motion in your shoulders, reducing pain and overall flexibility.
As this is a plate loaded machine, first place an appropriate amount weight for you on the bars either side and secure them and adjust the seat for what is most comfortable for you.
To start, place your shoulders back, and keep your back and head firmly against the pad. Grab the handles and push up in a continious controlled motion returning slowly back down.