Reenster,

The main reason is the price - because I am betting on longer odds horses I can usually get better prices on TAB than bookies. Unfortunately as you would know the TAB prices can change dramatically in last few minutes before a race so I wait until last possible moment to make sure the odds are still over my bet price - I never bet if the price falls below my cutoff. Also I can pick which TAB to place my bet with depending on which one is offering best price on my selection.

Another benefit is that I have avoided quite a few losing bets by observing my selection behind the barrier and as it loads and deciding it wasn't a good bet!