Hi Alan,
That's part of it, yes. Although the values can also be moved into the session queue in other ways (for example if you press a lookup button).
The primary reason for having the feature in the first place is so that you can "respond" based on what the user has entered.
For example, you can do immediate validation.
Or you can hide / unhide other fields on the form.
Of you can change the values of other fields. (For example, turn this on, means that thing over there gets turned off.)
And so on.
By default, for example, if you type a code into a lookup field (ie enter the code directly, don't click the lookup button) then the new value is sent to the server, and the server sends back the appropriate description in the comment field.
(Tip: try typing the _description_ into the code field and see what happens...)
Cheers
Bruce