This Assistant helps you to input your reminders in a much friendlier and easy manner, then it makes the perfect query and opens the default Google Now where you will only need to press the blue tick icon and you will be done.
This saves you the hassle to perfectly speculate the query which Google would want to listen to. You can also add a home screen shortcut on your device if your launcher supports it.
The Assistant does not act as a reminders app, but rather as a mediator app between the users and the Google Now reminders. The reason it uses Google Now reminder mechanism is that Google Now is a product made by Google and hence updated and supported regularly. Plus you get all the benefits like sync, availability, etc.
DISCLAIMER: This app or the developer thereof is not associated with Google Inc. at all. The purpose of this application is to assist the users in using the reminders functionality provided by Google Now. It is consistent with the legal requirements.