* Unauthorised key duplication. Think of a college dorm where residents all have keys, but you don't want them making copies, and keeping access to the dorm after they move out. So they use a design that most locksmiths don't have key blanks for.
* Basic keyholders getting masterkeys by taking locks apart. Think of a college dorm where they want every resident to be able to open the front door, but not each others' dorm rooms, and they want the fire department to have one key that will work for all the rooms. Because of the way masterkey systems work, a resident can take the lock off their dorm room door, measure some internal components, and figure out the cuts on the fire department master key. Restricted key blanks make this a lot harder.
Of course, this doesn't provide perfect security. Restricted keyways don't protect against someone with a CNC milling machine, or someone with a set of files and a lot of patience, or someone willing to break a window.
Modern buildings rarely install new masterkey systems these days - electronic locks are a much better system.