If you are doing work on switched circuits, like a trunked phone system, there will be discussion of the probability of end to end blocking and various other traffic quality measurements. Sooner or later Erlangs will be mentioned and it will be handy to know they are the unit of measurement of line use per hour (100% utilisation = 1Erlang).

For an out standing explanation, check out:

http://www.users.cloud9.net/~stark/ctappd.pdf