I can think of three good reasons - there are probably more but three is the number I stopped at - why you would pay the extra and buy a six month tax disk for your car.
1. You plan on selling the car before the year is up.
2. You doubt the car will get through its MOT in a couple of months time.
3. You can’t afford to buy a years tax this month.
Someone, not very far from here has bought a six month one, because they don’t want a pink tax disk on their car. I hope the DVLA decide that next year they will use that horrible orange colour they haven’t used for a while!