Europe detail...

Depends if you're in UK or Germany: some prefer dorsal (upper), some prefer ventral (lower) slit.

It helps because you maintain all the natural feeling in the glans because the foreskin is retained, AND the slit allowsa the foreskin to be pulled back readily for cleaning. There is a large population of people with phimotic (overly tight opening) foreskins that make it difficult to pull back.

In fact, the Japanese customarily leave their foreskin pulled back so that is somewhat stretched and slightly enlarges the glans.

http://www.circlist.com/anatterms/anatomy.html