It is much more affordable than most precious metals and is resistant to scratching, corrosion, rust and tarnish.
It is durable and can take being worn day in and day out. It is easy to clean - just use warm water and some dish detergent. If it's really dirty, use whitening toothpaste (just make sure that it doesn't contain silica). Remove any water spots and create a shine with a soft dry cloth.
It's more eco-friendly than other metals, creating less waste and using less energy in its production.
Stainless steel doesn't have the light, bright sheen of silver, which can be a positive or a negative - it all depends on what you like and the look you are after.
While steel might not be thought of as having the same prestige or cachet as the precious metals, I think the durability and cost outweigh should outweigh that, especially for frequently-worn jewelry.
"My heart is as true as steel." - William Shakespeare