If you've got something to say, my inbox is already open. Send it. A blank friend request just makes me guess.
the problem
It doesn't tell me who you are, what you want, or why now. It just sits there, waiting for me to either accept a stranger on faith or reply first to find out what this is even about.
Either way, you did the easy part and left me the rest. Clicking "Add Friend" costs you nothing. Figuring out your intent costs me a guess.
the difference
One of these I can answer immediately. The other one just gets left on read until I feel like solving a mystery.
why this, not that
The inbox is already open. If DMs are on, you don't need to be friended to say something. The permission you're waiting for already exists.
Context beats access. I can act on any message instantly. I can't act on a blank request at all, it just sits there.
It respects my time, not just yours. Sending the message up front means I don't have to reply first just to unlock the actual conversation.
a note
Adding someone you already know, or building toward a real friendship, is a fine reason to hit "Add Friend." This is about the specific case of a cold request with nothing attached, sent to someone whose DMs are already open. Where a message would have worked just as well and told me something.