Once you've gotten to the MVP stage, you should be tracking the key metrics, and those should be easily viewable in some sort of dashboard. Tracking up-time (or if the service is working) is a small part of what should be measured. When you're just starting off, you need to quickly figure out if your market hypothesis is correct. You can't do that without data, and a dashboard is both easy to build and helps make more informed decisions for everyone involved.
No one is arguing that you should build a dashboard before you have a MVP. Once that happens, you need to be measuring things.
They have variety of purposes. For e.g our webapp is an open book, we display live dashboard (https://my.infocaptor.com) of our very app itself because it is meant to be a dashboard platform
The main purpose of dashboards is to keep a birds eye view and take action when something is off.
Don't track metrics where you don't have time or resources to take action against.
Dashboard answers the question: Is my product right now working for my users?
Eventually it can also show trends in terms of resource usage, strange peaks, etc. But the primary function of dashboard is answering the question: "are my users getting the service?"
Additional note: dashboard works wonders for getting us - engineers - understand that there are real users out there right now and thus making us want to build even better product.