Creating custom msg message vs sending data on topic via string

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I was thinking that since using a custom message will have multiple fields means each field will have a key and values kind of data structure. This is similar to a dictionary data structure.
So are there any advantages of using a custom message over converting a dictionary to a string and sending it as a string, because both ways it will have the same amount of data as per my understanding? or when we use ROS custom messages it only transmits values and decodes those values at that node using a custom data structure format.

Only values are transmitted.

But that's not the point here.

A string has no meaning. If you receive a std_msgs/String, could you tell me what it contains? Other than some characters?

If you receive (for example) a sensor_msgs/PointCloud2, could you tell me what it contains? I believe you can: a pointcloud.

So semantically, it's clear that a sensor_msgs/PointCloud2 is not a string.

That's what the guiding principle should be when using ROS: clear, semantically meaningful messages.

Only values are transmitted.

Thx that's what I was looking for.
And I do agree that the topic should have a clear indication as you mentioned.
And if it uses less memory in doing so then we should definitely go for it, if your topic is for internal communication only. Correct me if I am wrong.

It's unclear to me what "internal communication" means or why one should eschew good semantic practice in them.

internal communication in terms of when you are not sharing those topics publicly