In the salad days, before the advent of mobile devices, people consumed cloud-based services via their web browsers. Accessing a particular site was a matter of typing the URL, or better, clicking on a link. Search engines then simplified content discovery, and so all you needed to do was type a search text and a list of most relevant links popped up on your screen.
In this era, the metrics most important to the marketer are:
(a) number of users visiting the site at daily/weekly/monthly granularity
(b) number of new users visiting the site in various time intervals
(c) number of repeat users visiting the site in various time intervals. The first sought a healthy mix of new and repeat users: the second an indicator growth and the third an indicator of sustenance.
With the emergence of mobile devices, this website-based consumption has slowly given way to app-based consumption. A user first needs to install the app on the device before he/she can access the content provided by the app. Memory and manageability constraints dictate that a user cannot have more than a handful of apps on his/her device.