Why would you want a judicial clerkship after law school?

Looks good to future employers. You can get paid while not yet being licensed. You get to know what lawyers that practice in that court are good and which ones are idiots. You get intimately familiar with a few areas of law. You get more experience writing than you would otherwise. You might end up with a lifelong friend. I have clerked twice. The second time, that judge became a great friend. Even though I now leave thousands of miles away, he comes to see me a couple times a year and likewise travel to see him. As there is at least some possibility that he may end up as governor, that is a nice friend to have. The first judge I clerked for ended up as the boss of a huge federal agency and he asked me to come along with him as a sort of chief of his legal staff. I declined but it was still pretty cool.