It just gives you a year of studying e.g. biochemistry, genetics, PDN, etc from the natural sciences tripos normally (occasionally a different third year course from another tripos).
As stated, all medical specialties in the UK have now removed the extra points that were previously available for an intercalated degree (on the basis that not all students had equal access to these, financially and practically since not all unis offered intercalation) so there is no professional benefit directly from it.
Part II (the intercalated year) is mandatory for all medics at Cambridge on the standard entry course unless they already have a degree.
Intercalation is not that uncommon in medical degrees, although not all offer it. However now it's really purely a matter of whether you want to spend an extra year studying something in more depth, usually a non-clinical basic science subject at most medical schools, since you can no longer gain portfolio points from it.