Maybe the difference is this.
Boiled rice gives you one type of rice. It is relatively easy, but I know a lot of people tend to want the softer fluffier rice cooked using the absorption-method.
Rice cookers are doing absorption-method cooking. This is slightly more technically complex - the exact ratio of water to rice is important, as is controlling temperature.
I consider myself a reasonably competent home cook. I cook rice fairly regularly on a stove top, but getting the exact ratio of water to rice correct is something I find is very easy to mess up.
Additionally, my current stove is fiddly, and hard to control the exact temperature. Sometimes I will end up with a situation where the rice has fully used all of the water, but is not cooked through, other times I will end up with rice that is fully cooked, but is still quite wet.
When trying to time this with other dishes, it's painful.
I don't have anywhere convenient to store a rice cooker, so I don't have one - but when I renovate my kitchen in the future it will be one appliance I end up getting.