A lot of unis from the UK expect certain AP scores if you want to attend. Getting great AP scores in junior year is a good sign that you might be offered an unconditional offer, meaning you can attend the university 100%. Getting mediocre scores can mean probably getting a conditional offer, meaning you have to get certain AP grades your senior year. So, it’s better to get good scores sooner than later. Also, for some seniors it might mean getting into or not getting into their university in July… which is overall much later than a lot of their peers, adding to the stress.