Although the above comment MAY well be true in some cases, but there is generally a good reason for such recommendations too.. People who know other people who have worked well in some projects in the past and have good references, companies prefer them over getting someone completely new who might not fit well.
In IT, as much as any other job, its not all about knowing the technical stuff, its to do with "Soft Skills" too (i-e dealing with people, conflict management, project management, communication skills, management skills etc etc) which some candidates lack in.