I read a guide on self-editing your manuscript, and one of the recommendations is to consider providing your side characters with their storyline instead of being just one-dimensional helper. What's your take on that? What if you have many side characters? I couldn't find similar discussions in here, so I apologise if it has been brought up before.
I don't think you necessarily need to have a "side story" for them, but I do believe that any characters who play a significant role in the story should be fleshed out with a goal, motive, and conflict. If they have these three things, they'll develop their own story arcs that hopefully will be resolved by the end of the story.
You don't need side-plots for every side-character, but it does add depth to your story to have other characters grow in their own ways alongside the main character. They're people too, with their own problems and aspirations that don't necessarily revolve around what the protagonist desires.
I don't give them to every side character but I like to give as many characters as possible a life outside the book and the plot. It doesn't have to be in depth, just hinted at.
You don't necessarily need to give them the depth of a main character, but they need to be at least to somewhat understood in relatable sense. Otherwise they are simply a tool, or an emotionless robot. And they're predictable, which can be an even worse sin. Personally, I at least try to give them a small personality, and a few life goals. It at least gives a bit of direction for them and allows the writer some confines to work within for what the side character finds acceptable or is comfortable doing. Make them real, just don't waste too many words or too much time on someone who ultimately isn't as integral as an MC.
I think the thing is that you want to make all your characters seem real for the sake of the story. You don't want any side characters to seem like props. Giving them a side story helps round them out. If a character in your story is just there without a bit of their own story, why are they there? If they have a role in the story, give them life which will help your overall story. As for having too many characters, that could be a problem, especially if there are a lot of them the reader knows little about. I'm all for secondary characters having their own stories. Flesh them out and tie them in. It can add quite a bit of depth to a story.
Giving them their own storyline in your head so you have a point of view for them, yes. A full storyline in the story? No. But echoing what EF Mingo stated above, a personality and some life goals helps give them dimension. It also gives you something "to write to", which is helpful in creating well-rounded characters. It's easier to write them when you know who they "are".
I try to dedicate a chapter to introducing each side character that the reader has to remember. That way, one can easily recall the story that gave the first impression and the information becomes less scattered. If the side character is not important, I may use a simple description like "The woman in the white stola", so that I won't run out of different sounding good names. If it turns out that the character wasn't needed for the plot, it can be removed later to keep it clean an memorable. If it's slightly difficult to remember all characters, it will pull the plug on any drama because nothing will feel unexpected for a certain personality.
I think it's only needed if it adds value like, adds to the plot, character relationships, or general interest to the idea/theme.