To begin with, the design of the main characters are design the earliest. Usually, far earlier than the anime series. So, the background characters are drawn later.
Of course, without these background characters, it would look out of place. There would either be silhouettes (Bakatest), or even completely void of them (Bakemonogatari). Sometimes, they are included, but are not animated (K-on) or very little (Railgun).
Unfortunately, in images like these, there are errors
Take this screenshot for example: in the left image, there are details included like the posters or the duty roster next to the chalkboard, but that girl closest to the teacher on the right is wearing a blue tie instead of a red ribbon. In the right image, the second and third girl from the left have the body proportions out of place: their head is smaller than the body! Of course, the right image is displayed quickly.
Of course, the background characters has to match with the style used for most of the characters, but sometimes, they would just use the same face, but with different hairstyle, outfits, and body language.
There are those where the background characters look a lot better than primary and secondary characters.
There are those that add to the realism of having a background character reacting to what secondary characters are doing to them, even if it does not have any actual outcome. Here, a girl (in uniform) is forced out of her seat by Harima Mika (wearing white). She is forced to move to the empty seat behind the main character (Mikado). This all happens while the main character is narrating about Seiji (guy next to Mika) who finally turned up in school and switches focus to Sunohara Eri who is looking at Harima and Seiji and is seated at the front left.
It’s sad that these background characters that look a lot better than the main characters don’t appear much and won’t be remembered as they appeared for only a while.
Oh, and one last thing. Can you guess who this is?