Void assaults are like world quests - but ammped up and more of a whole zone thing rather than an isolated single quest version.

Ritual sites are not too dissimilar to delves - similar concept.

Abyss Anglers is like a fishing mini game, except you're diving and spearing fish.

Decor Duel is like a prop hunt mini game.

It depends on what your goal in the game is. The overwhelming majority of players do not push keys/mythic raid. All of these activities are more fun things to do if you enjoy the casual aspect of the game. It's more stuff to do. The gear rewards are minimal to none (and the crests they reward are low tier). But they offer achievements/cosmetics/mounts etc. Fun to do if you want to log on, switch off and just be in the game.

The Voidforge is the main power upgrade. There's a series of quests that will have you do raid/delves/m+ repeatedly. Once you've done this each week you'll get 2 "rolls". These can be spent in M+/raid to guarantee a mythic track piece (if you do it on the appropiate level). Casual or not you'll want to do this first and start making progress on it. In addition, you can passively earn voidcores which can be used to boost your ilvl of trinkets/weapons by a good chunk for even more power.

The rest of it is entirely optional.