It is almost universal that open-world games like Dying Light - and particularly survival horror games of this sort - will feature a Collection of side-quests. If you actually add up the time and effort that these take you will discover that the side-quests in Dying Light equal more than half of the actual game play for the single-player story play-through.
Fortunately they not only add to the story - filling in all sorts of details - but they also for the most part add more entertainment in a game that to be brutally honest tends to take itself way too seriously!
The humor that was part of the Dead Island series (also by Techland) was a key element in the making of its personality. In Dying Light they intentionally moved away from that sort of approach - saying that this was what they meant Dead Island to be all along.
We don't know about that - but what we do know is that adding these side-quests was a very important decision on the part of the wizards who created this game.
You may also find it of interest to learn that they also created a significant Achievement / Trophy for the game specific to the side-quests. It is called The While Story (100g) Finish all side quests - and as you can see it is worth one hundred g!
The only useful advice that we can provide is this: flag every and any side quests that you encounter. There is no limit to the number you can have active at any one time, and having all of them that you find active means you are hardly ever not near a quest action!