Yale knows some of its applicants will be highly coveted by other colleges – by issuing a LL, they hope to come to the front of the line in terms of courting and wooing the object of its affections. Many of these will be very strong STEM candidates whom Yale feels will be a good fit. For general info on LLs: http://talk.collegeconfidential.com/yale-university/804816-quick-facts-about-likely-letters.html