You're right - there are many overlapping features between smart phones/tablets and smart displays. But many of the features on smart displays are enhanced for the purpose of being a home assistant imo. For example, I don't need to tap my phone to activate the Google assistant function when using the smart display and I can call out commands from much further away. Since my Lenovo smart display sits on my kitchen counter, this is useful because my hands are usually busy (chopping/stirring, oily/sticky or even carrying my kid, you name it haha) and I wouldn't want to keep touching my phone to read the recipe/google for a ingredient substitute. I guess it's the constant availability of the device to attend to my needs that makes smart displays appealing for me.
My house didn't have much to work with in the beginning but we slowly added more smart plugs and lights around which is probably why we benefit from using smart assistants/displays more and more. Definitely made things more convenient for us!