Transactions – Why is the privacy of watch tower services usage increased with unpredictable engagement IDs?

in the open channel In the BOLT 02 message, the following statement is about the different basic points.

The various _basepoint fields are used to derive unique keys as described in BOLT No. 3 for each commitment transaction. Changing these keys ensures that the transaction ID of each engagement transaction is unpredictable to an outside observer, even if a commitment transaction is viewed. this property is very useful for preserving confidentiality when you outsource transactions with penalty to third parties.

I'm asking about the last sentence. Why does this contribute in particular to the confidentiality of these services? I was thinking of increasing privacy by using several third-party monitoring services, not just one. In case I would use one, they would know anyway the complete history of the state of my channel.

One thing I guessed was that once I knew of a commitment, I could calculate all the TXs for all possible channels. But I thought the signatures depended on the number of exits and the txid depended on the signatures.

Do I have a misconception or is the reason for the increased confidentiality another that I do not see?