I went through an assessment day recently and achieved the highest score and was the key performer in the group. That was down to encouraging the others on my team to contribute to the group task and because I followed the verbal and written instructions correctly, whereas 5 of the 7 applicants failed to read the directions and so failed the task.
I showed leadertship skills without being domineering and I helped my group plan and organise the presentation by suggesting everyone took an equal role and by asking them for input.
In the Role Play I showed confidence and an understanding of the product and the customers expectations .
They are a good experience and the key is to be noticed for your team work not for your ego.