My DS is only going because he can't make up his mind. VanHelsing. My DS1 went to two a few years ago as he was trying to decide which to use as insurance. They are basically a subject specific open day. Not usually more informative than a regular open day, if you attended the subject talk. Sometimes they may 'interview' you. It's a marketing thing, basically, they are having another stab at selling the uni/course to you. Not compulsory, nor necessary unless you want another look.
Hoghgyni soon, I'm sure. 