That would not work for Gmail accounts, since Gmail sever saves outgoing messages in Sent folder (on the the server).
Why not create specific rules/filters on the server to do what the user wants?
1. A large world exists beyond Gmail.
2. Using the said extension in Thunderbird, or alpine or mutt, even with Gmail accounts you can save the outgoing messages to the particular folder. Yes, Gmail will create a copy of (or, actually a duplicate link to) that message, but so what?
3. Not all e-mail services/servers allow users setting up rules/filters.
But you cannot create a rule/filter for all different cases even where that can be done. And if it is a brief exchange (1-2 messages each way), it is much easier to save the message from the client, then by writing a new rule on the server.
Think of why "Saving a message to a folder" exists for incoming messages. The same reasons apply why that would be useful for outgoing messages. Piling up all outgoing messages to a single folder is just one of the models. And, in large-volume accounts, it is far from the best.
In many cases, it is much easier to be able to save a