Complex sentence: The press is primarily intended for the development of the shoulders, but because the lift is performed standing, it also develops the arms, the abdominals, the obliques, the costal muscles, and the back, which all act as stabilizers.
Split into simpler sentences: The press is primarily intended for the development of the shoulders. But because the lift is performed standing, it also develops the arms, the abdominals, the obliques, the costal muscles, and the back, which all act as stabilizers.