Because knowing even a little bit about art or a little bit of german makes you a more interesting person? You don't have to be good at something to find interest in it.
Which is fine if you have a motivational/interesting teacher. Yes, you may get something out of a subject you have no ability nor interest in.
Unfortunately, not all teachers are like that.
A lot of teachers are pretty shit at motivating in arts, music, drama, tech etc. They facilitate those who have an interest or aptitude, but if you don't, the lessons can be pretty boring. All I remember about art is constantly drawing/painting on paper - it was fine at primary, but it didn't move on in secondary - I wasn't particularly "arty" so my work always looked childish and pretty crap compared to some of the others, so I dropped it as soon as I could. From what I gathered, it got more exciting once exam options had been chosen when they started doing photography, screen printing, etc. Perhaps I was unlucky to have a lazy art teacher?
Same with my son - he was really looking forward to his "tech" lessons as the open day showed all kinds of robotics (inc lego mindstorms solving a rubik cube), 3d printer, laser cutter, and a computer suite with cad/cam stations, etc. First year, they made the obligatory wooden fish and metal trowel - he wasn't too good with his hands, so they were pretty amateur, but teacher was very complementary and said things would be better once they started using the "tech" as it was more about design further on. Second year came and went, slightly better as they started on working with plastics - a vacuum formed clock and a torch, but it was still all by hand. Third year, back to hand tools to make a wooden box. He had all but decided to drop tech and do something else for GCSE, but they had an "options" open day, and all the "tech" came out again, with promises of using the cad/cam to design and make the project, with only minor finishing off by hand (such as sanding/varnishing etc). Son actually got quite excited and chose it as a GCSE option. Big Mistake. They spent the first year using the cad to design the project, so he spent hour upon hour at school and home doing 3d designs, detailed plans, etc - really impressive stuff. But then when they came to start using the laser cutter, CAM equipment, etc., the teacher went off on sick leave and the cover teacher didn't know how to use them, so they were all told they had to make it by hand - son now wishes he'd just dropped it when they told them, but we encouraged him to carry on, and the teacher promised to help. In the end, he was on his own, and his project turned out as if a 5 year old had done it. All the while, the "tech" room was kept locked, the laser cutter, 3d printer, etc all just sat gathering dust. What annoyed him more was that other pupils had parents, grandparents or uncles with the "workshop" in their garage and were producing really good stuff (with lots of help!!). He actually scored nearly full marks on his theory/design work, but in the lowest band for the finished product. If he'd known he'd be making it by hand, he'd have designed it differently, with simpler shapes, simpler joints, etc. The school badly let him down and it's now put him off "tech"!