With spirals, you move the wood into the blade following the lines without having to make turns with the stock. Excellent for longer pieces that are too long for the throat of the saw.
That said, corners are not as crisp, nor are the cuts compared to flats.
I used spirals for a while, but my DW788 with the Pegas clamps does not do well with spirals. I need to make some adjustments to my clamps which will be after I finish ornaments.