There are many reasons why having a Smart Home Hub makes sense, but the big ones have to do with security, data storage, and network traffic.
Let's start by showing the network map of a typical Smart Home with no Hub.
Now here is an example of the same network using a local Hub that still allows remote access:
A Smart Home Hub will not only cut down on the internet data usage, but it also eliminates the need for installing multiple apps to control your devices. Don't even get me started on APIs that are meant to get APIs to talk to other APIs.
Choosing devices that support local control and allow the option of an API will make things work smoothly down the road while API dependent devices will limit your options. The real magic happens when you get communications standards involved like Z-Wave, MQTT, and Zigbee.