Stuff like bulk priming (still can be done without secondary, tho) and racking onto additional ingredients is a valid reason for a secondary
Most of the time though if you're just making a simple ale/lager, a single fermenter will do just fine
I use a single fermenter and have crystal clear beers, others prefer to secondary
whatever floats your boat, really (as long as you accept the possible risk of infection and oxidation with a secondary)
*edit*
Go easy with the treacle, it can be VERY overpowering