> transient-mark-mode just allows you to see what you're selecting.
Yeah, no. It's a modal state, and since Emacs doesn't support modal interfaces directly, it touches tons of separate commands (anything using `region-active-p`) and means it's tremendously difficult to find out all it does!
Many commands change their behavior when Transient Mark mode is
in effect and the mark is active, by acting on the region instead
of their usual default part of the buffer’s text. Examples of
such commands include M-;, M-x flush-lines, M-x keep-lines,
M-%, M-%, M-x ispell, and C-x u.
To see the documentation of commands which are sensitive to the
Transient Mark mode, invoke C-h d and type "transient"
or "mark.*active" at the prompt.
The reason selection with a mouse also triggers it is because all these commands would not DTRT unless it did, given Emacs's approach of forcing mice to fit into the system designed for keyboards.
