I think more importantly 'custom timers' can be used to time a GROUP of actions. For instance when you click on a submit button, sometimes 2 or 3 or more actual posts are being sent to the server. So if you wrap them all in a custom timer, you can see how long it takes for the entire set of actions to occur rather than each individual one. Also with custom timers you can make the names more legible than the default SP page or form timer names. A standard SP named timer may be "OrderSubmit.aspx (#21)" Whereas you could name this "Submit an Order with standard shipping" or something that is more relevant to your target audience.
WebURLPostBin(some dynamic post to say a web service)
//a redirection maybe
WebAddURL(A nice pretty little hourglass.gif)
What I've trained myself to do is WHILE recording, I start a timer before clicking/submitting something. And then stop that timer when I'm sure the page is complete.
The Timer or rather "Custom timer" in SilkPerformer are same as the "Transaction" term is used in Loadrunner. While "Transaction" in SilkPerformer is same as "Action" in Loadrunner.
Its bit confussing when you switch from Loadrunner to Silkperformer or vice-versa.