Maybe you want to test the rules before jumping in "whole hog", so to speak. There seems to be a near limitless selection of sourcebooks, scenarios and campaigns for the system.
Maybe you're like me, and you have had the game for years, played it once or twice, and you would like to refresh your memories without rereading the rulebook again. Because maybe 28 pages including the scenario goes down that much easier.
Maybe you like to have a free collection of quick starts. I know I do. :)
The Call of Cthulhu Quick Start is an excellent example of the type of product Chaosium puts out. Grab yourself a freebie ;)
From the blurb:
Horror Roleplaying in the Worlds of H.P. Lovecraft
The Great Old Ones ruled the earth aeons before the rise of humankind. Originally they came from the gulfs of space and were cast down by even greater beings. Remains of their cyclopean cities and forbidden knowledge can still be found on remote islands in the Pacific, buried amid the shifting sands of vast deserts, and in the frigid recesses of the polar extremes. Now they sleep — some deep within the enveloping earth and others beneath the eternal sea, in the drowned city of R'lyeh, preserved in the waters by the spells of mighty Cthulhu. When the stars are right they will rise, and once again walk this Earth.
Welcome to Chaosium’s Call of Cthulhu Quick-Start Rules, a booklet that collects the essential rules for Call of Cthulhu and presents them in abbreviated form.
This book comprises a basic roleplaying game system, a framework of rules aimed at allowing players to enact a sort of improvisational radio theater—only without microphones—and with dice determining whether the characters succeed or fail at what they attempt to do. In roleplaying games, one player takes on the role of the gamemaster (or Keeper, in Call of Cthulhu), while the other player(s) assume the roles of player characters (PCs) in the game. The gamemaster also acts out the roles of characters who aren’t being guided by players: these are called non-player characters (NPCs).
From its origin, Call of Cthulhu was designed to be intuitive and easy to play. Character attributes follow a 3D6 curve, and the other Call of Cthulhu mechanics are even simpler. Virtually all rolls determining success or failure of a task are determined via the roll of percentile dice. This means that there’s less fiddling with dice of different types, and the concept of a percentile chance of success is extremely easy for beginners and experienced players to grasp.
Use this booklet to play Call of Cthulhu immediately. Games rules, pre-generated characters, and a classic adventure are included. You will need to supply your own dice.