I mean, when I write at least, I try to make everything tie back into itself. I'll admit that there have been many stories for certain characters that have just been 'because I wanted to do it', but I always try to give some sort of plot reason in the end to it, tying it all together.
For example, (Spoilers if you're reading my story) I wrote a certain arc where I allowed my main characters to attend a school. Why? Because I thought it would be fun for these evil demon lords to attend magical academy like everything was normal and then all of a sudden whip out their overpoweredness. However, in doing this, I ended up introducing a character who was immensely important to the progression of the series, and developing said character throughout the arc, using the events of the arc which led directly into the main plotline.
Buildup is very important in storytelling, and if you just spring things on your viewers, it will leave them with whiplash. On the other hand, sometimes springing things on them can achieve a shock effect. I personally really enjoy building up a character in a direction that seems obvious, then to spring the reality on them. Like when I'm writing a fight where one character should very obviously lose, I often show them growing and developing, using new tactics and improving their abilities throughout the fight. Maybe they even unleash some new form or power level, and gain the advantage in a moment of victory.
Only for them to realize that they aren't the main character, and that the entire fight was just the opposition messing with them.
But as far as buildup of a character, I think that you need to show enough so that people understand 'who is this character'. This doesn't have to be through explicit filler 'arcs'. Each scenario that a character is in can show you who they are. Whether these little scenarios are related directly to the main plot.... I don't think they have to be. Because showing who that character is, at that point, is the main plot. Small scenes of simple dialogue, little fights and secondary conflicts, these are all important in building up your characters and how they function with one another.
At the same time, I have done entire arcs just to develop certain characters. And that's perfectly fine. Of course, these arcs do have long lasting effects which directly move the main plot forwards, so it's all good.