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Using a stackmat makes your time a little slower, so it's a more realistic way of timing yourself when you compare it to being timed at a competition. So if you want people to take your times more seriously, you should use a stackmat, as it's the official way of timing people.
I personally don't care that much about this, so I use keyboard+CStimer. I'll figure out the stackmat once I get myself to a competition. I might even buy one at some point.