Google Assistant is a Googles immitation on Apples Siri, Mircrosofts Cortana and Amazons Alexa. The app is currently on #2 for producticity app on the play store. No surprise there. The virtual Assistant is inbuilt on most of the devices running on Android Marshmallow and above. All we need to do is press and hold the home button or say OK Google to open the assistant. The Google Assistant app is for devices that dont have in built assistant or devices running on old android operating system such as Android 5.0 or Lollipop phones.
You are not required to download Google Assistant app if it is already baked into your phones launcher. The app provides one touch access to launch the Assistant. It really does what you want the Assistant to do for you. For instance, it can make calls for you, send text messages, play music, set reminders, navigate places, weather and news updates, etc.
Though my phone has the in built assistant, I downloaded the Google Assistant app for the heck of it. And rightly so I realised I didnt need this app as I can launch it from my home screen without downloading the app. As far as the app is concerned it is quite responsive, and has a minimal lag. I dont use my assistant more often unless I am too bored. So I did ask few things like, whats your name? Favourite color and what it eats. Funny, but assistant has all the answers. His/her name is Google assistant. Favorite color is blue, red, yellow and green ( typically the apps colors) and it said it is always hungry for knowledge. I also checked the weather, read a joke and read a news article using it. Since Google is not a God so it also doesnt predict accurate weather at times. But I must say it is a great companion to kill the boredom.