Semi-acoustic guitars are designed to be played both acoustically and electrically. When playing acoustically, the guitar's sound is produced by the vibration of the strings against the soundboard, just like a regular acoustic guitar. The hollow body of the guitar amplifies the sound, giving it a louder and richer tone than a solid-body electric guitar.
However, semi-acoustic guitars also have the advantage of being able to be played through an amplifier, which allows for a wider range of tones and effects. The electrical signal from the pickup can be amplified, distorted, delayed, and more, allowing for a variety of different sounds.
So, while you can play a semi-acoustic guitar without an amplifier, you may be missing out on some of the benefits that come with plugging it in. If you're looking for a guitar that can do both, a semi-acoustic guitar is a great choice.