The Way it goes....Mainquests give you chaptergoals, at least one of the goals is mandatory for the chapter to progress, some should be done other choose a path or the faction(s) you wan't to help/fight/...

The Mainquests have linked Sidequests that can be different path to reach the goal or things to make it easier to reach the goal, like finding a secret passage to evade a ambush on your way or ambush the ambush yourself.
Most or all fo the linked quests could be ignored in theory...you could just fight your way through the ambush but on tactican+ it can be quite hard and you will do most of the linked quests.

Then you have the free Sidequests, these are not directly linked with the goals but can influence them (making allys/enemys,...etc) on classic and below you can ignore them if you wish but you will miss some interesting storys/lore(and loot+exp;).

Long story cut short, classic and below allows you to ignore many sidequests otherwise you will do most of them to gather strength at least during the 1 act and the beginning of the 2.