In any market, the biggest attraction for people is the accessories you can get for each device. The Apple Watch for example, has different charges, bands, straps etc. The same is also true for smartphones. The primary difference is that sometimes an accessory for smartphones are actual devices themselves. Here are some of the more popular accessories:
Although considered an accessory this is actual a necessity. It protects your screen from scratching and damage in general. New generation screen guards are glass so there no loss of feel when using one.
These serve two roles. First is personalization. You can make your phone reflect your personality simply by changing the cover. The other is protection. Some of the more durable cases can even handle drops from a decent height.
These are devices in their own right, but in some cases need to be paired with smartphone to be effective. In that sense they are considered phone accessories. Take the mi band for example: it tracks sleep, walking etc but is useless without a phone.
This is where the Apple Watch and Android watches come in. They are mostly used for notifications from the phone, and to a lesser extent for the time. Most people buy them as fashion accessories.
Last but not least are holders like Google cardboard which convert the phone to be able to act as a VR display device. The most popular version of this is the Gear VR sold by Samsung as accessories for their phones.