It doesn't. The concept was that somehow a uber unit could be made by further kicking the crap out of people who had previously had the crap kicked out of them---so, in their scenario you have a SEAL who has been through BUDS and you then send him to another Hellweek because? So we send people to the Q course and to Ranger School because those courses teach a different method of fighting and one which SEALs need to be knowledgeable about--and they are difficult courses but there is benefit.
That said, units frequently work together in a composite manner---but it is to either derive some benefit from unit specific skill(river survey for GBs in Afg) or to increase manpower (supporting with blocking positions/snipers etc).
I believe that the script was originally written with O'Neil going to BUDS but was not supported by the Navy(yes, the Navy gets a vote) because it could be construed as the SEALs being sexist and out of control--sot they created a fake unit that everyone knew was fact so there was nonone to look bad--per se