There are several reasons why people may choose to use glass or metal dildos:

1. Non-porous material: Glass and metal dildos are non-porous, meaning they do not absorb bodily fluids, lubricants, or bacteria, making them easy to clean and sanitize. This is especially important for people who share sex toys or have sensitive skin.
2. Temperature play: Both glass and metal are excellent temperature conductors, so these types of dildos can be quickly heated or cooled before use to provide a unique and stimulating sensation. The temperature change can also help increase blood flow and sensitivity in the genital area.
3. Smooth and firm sensation: Glass and metal dildos are known for their smooth and firm texture, which can provide a different sensation than softer materials like silicone or rubber. This can especially appeal to people who enjoy more intense or direct stimulation.
4. Aesthetics: Glass dildos, in particular, are often handmade and can be quite beautiful and artistic, making them an attractive addition to a sex toy collection.
5. Long-lasting: Glass and metal are durable materials that can last a long time with proper care and storage, making them a good investment for people who use sex toys regularly.

As with any sex toy, it's important to choose a material and style that feels comfortable and enjoyable for you, and to use the toy safely and responsibly.