It depends what your motivations are. I realise this is vague but say as much as you need to, and no more.
Also AFAIK WC are the ONLY firm to guarantee/require an overseas secondment - this is very different from a firm that runs them but maybe only 20% of trainees get to go in any given year. Apart from the fact that going abroad is cool and fun, also think about what it might mean about the firm's values - is WC committed, in a way others aren't, to training truly international lawyers, to building a truly international brand, to running a truly integrated global operation underpinned by strong relationships between offices?
I didn't think of it this way, this helps so much thank you!!