Two reasons. The one is like to say is for the local control. But the truth is, while I'm a geek and love playing with this, my ever increasingly tired old brain is sometimes getting overwhelmed with just remembering what I did, and what I used to do it! Lol
So to try and simplify things, I wanted to try and do everything, as least as much as I can, via hubitat directly. My wife however, is not a geek like me, and as we're in our 60's, I want to make everything as user friendly as can be. She doesn't want to use any app to control things, only vhoice commands, so I'm using Google home to try and control everything. But what I didn't want to do, was to have it run any of the automations, again, just to keep it all straight in my head of what I have going on. Doing it this way helped, in that at least if I have an issue with the front porch lights not turning on for example, I know where to look.
Here's another newer example. I have the roborock curv (which was lot of $$$ but with 4 cats, its one of the best things I ever bought) I used the custom app someone made and created a virtual switch that run one of the custom cleaning routines. Now she can just say "clean powder room" and Google turns on that virtual switch, which then in hubitat runs a routine to clean our guest bathroom where one of the litter boxes is. We have one of our four that will go in there sometimes and have a party throwing that freaking litter all over the place.
There are just so many other things I'd like to be able to do, as we don't live a very normal life. By that I mean, she get inspired at odd times, and if the weather is nice, I'll find her outside at 3 am on the front porch working on book she is trying to write. I'm now playing with these presence sensors I bought trying and racking my brain on how I can write some rules for all the different things we do at different times around here. It's all good fun 
Btw, I'm always dreaming of what I can do next. Like if I can figure out how to hook up a sensor that will let hubitat know if that one cat goes in there, I can have it run that routine. Someday... There will be an easy way to do it. The thing is, I know how too, but there isn't access to the API from what I've read. We have a litter robot in there, and it has a scale built in that let's it's now which car is using it, based on their weight. Now if they would ever open that up, someone who is more of a geek than me could build an app to access it. It sure would be fun to use it if it happened.