Only is a powerful, defining word.
Only sets the record straight.
Only lets everyone listening know the speaker's sense of value as much as it lets everyone listening know the speaker's sense of what is relevant.
I hear only used in schools and among educators and students in one predominant context: only reading.
As in, "they are only reading."
As in, "oh, your kids are only reading...I'll come back."
As in, "we/he/she/they/you only read in class today."
I remember being in elementary school and being asked without anyone really truly expecting an answer, "Oh, you're an only child? You must be spoiled." It wasn't really a question. No one wanted to hear what I had to say about only being an only child. And that used to, well, break me down. So, there were times when I wondered if I missed something by being an only child.
When people called me an only child, they meant it. When the term only reading is used, people mean what they say. Perhaps they don't intend it to be quite so harsh, judgmental, damning, belittling, or destructive.
But it is.
Only is a powerful word.