Why is a comma needed here? Before "some"

My teacher said that in this sentence: "She noticed that on one of the pages some information about the painting had been written." I should put a comma before "some". I don't know why and I don't have a way to ask her that right now. Could someone please explain this to me? Thank you in advance.

A comma before ‘some’ alone would not be correct. You would need then another comma before ‘on’, i.e. you would have to enclose “on one of the pages” between commas. This is correct, because the sentence makes perfect sense if you remove “on one of the pages” altogether. On the other hand, no commas is also correct and makes for more agile reading.