What's the purpose of syrup in toffee apples? The toffee apple recipes pretty much all have syrup in them.  The ratio of syrup ranges from low to almost as much as the sugar.

My question is: What is purpose of the syrup? If it's just for flavour, then how does it affect the flavour?

I realize it's easier to get the caramel going in the pan with syrup in the base, but if you add water (which most recipes say, and it boils off anyway) that makes it easy enough to get going without burning the sugar.
The bane of sugar syrup or caramel making is unwanted crystalization. A few stray sugar crystals, a premature stir, and your caramel gets grainy instead of smooth.

Corn syrup is an invert sugar (glucose), which can prevent this. Alternatively, a bit of acid (a few drops of lemon juice, a pinch cream of tartar...) will break some of the sucrose (plain sugar) into fructose and glucose. 

Do you have to use it? No. It's mostly a safety net. If you follow proper procedure, like not stirring, washing down stray sugar crystals from the walls of your pot..., you can make your caramel apples without syrup. And there are recipes without syrup to be found on the Internet, for example here at Serious Eats.