"Comment for reach" as a comment to a post.
Just don't do it. What this really does is indicate one (or more) of these problems:
You're lazy
You lack understanding of context
You suspect that no one really cares what you think
You don't respect time (your own time or that of a reader)
If there is something you think others should see/read/watch on LinkedIn (or anywhere) say why that is! Trying to expand exposure without context not only runs the risk of a person thinking one of the above things about you, but it can backfire. How? You're leaving it up to someone else to try to interpret why something is important to you. What if they take the exact opposite meaning from a post? Maybe an AI program does this for you. A meaning you don't agree with? It leaves the door open to an assumed endorsement when intent may have been to point out a bad example (in your opinion) of ________________.
The last bullet point is most damning professionally. You invested none of your time to explain why it might be important, so why should a reader spend one second trying to figure it out? Don't make people play detective when it comes to why you communicate.
There is actually a worse sin when it comes to this misguided practice. People that comment "CFBR" (Comment for Better Reach). Now you're hoping someone knows what this means? Or they are going to look it up?
You find a post you think that is worthy of sharing? Say why! If a person is connected to you, they have demonstrated some interest in you. Enrich their time (and your's) by adding a quick note on why you're sharing that information. Otherwise, what you're really saying is "Remove Connection" and "Unfollow" because "I don't value your time".