While the main story of Diablo: Immortal is easily played through in a couple od evenings that doesn't mean there would be shortage of things to do. Even the story kinda continues as it's just a setup for the multiplayer part of the game.
First there is the exploration part. While this is nowhere near the biggest gameworld I have seen there is still plenty to explore before you get 100% mark on all the zones.
The zones also have different quests that pop up randomly within the zone. There is surprisingly good diversity of the quests, not all of them are the basic "kill x monsters" type ones. In addition there is also daily quests and an npc that gives directions to three random zonal quests per day.
There are also events that happen on set interval. There is actually two types of events. Ones that happen quite often like boss spawns and others that happen only on certain days at certain times.
Given all these different things (and these are just some examples of all the things that you can do within the game), it's easy getting sidetracked while doing something and encountering an ad hoc quest to do along the way.