I think Jeff did a more than adequate job of explaining the need for block brakes.
I should also add that rides are frequently engineered so that the proper ride speed can be achieved by a train coming off of the block brake, and for that reason, block brakes are frequently also used to trim the ride...so that the train will be going about the same speed through the end of the ride whether or not it had to stop at the block brake. As for the braking situation on Cedar Point coasters....
Blue Streak has no trims, but a long safety brake uptrack of the station.
Cedar Creek Mine Ride has two trims positioned to limit the train's speed aproaching the lift hills; it also has two block brakes which are normally open, and a block brake uptrack of the station normally used to trim the train approach.
Wildcat has NINE block brakes on it, all of which are normally wide open.
Corkscrew has a trim brake just ahead of the lift hill, and a series of block brakes approaching the station.
Gemini has a block brake on the 'far' turnaround which is normally used for trimming, and a set of trims just before the helix finish.
Iron Dragon has no trims on the course, but there is a block brake at the base of the second lift which is usually used to trim the lift approach.
Magnum XL-200 has a trim brake on the turnaround, then two block brakes at the end of the ride.
Disaster Transport has two governor wheels at the block brake positions...sometimes these are brakes, sometimes they are boosters.
Mean Streak has a lift approach trim, three trims on the first drop, and a mid-course block brake.
Raptor has a mid-course block brake used for trimming.
Mantis:TCFKAB has a trim on the first drop, a mid-course block brake, and two block brakes behind the station.
Woodstock Express has a block brake at the end of the ride because while Cedar Point has only one train, V???? ???????? ?? ?? ??? ???? ?????????????? ?????? ??? ??? ??????? ????? ??? ?? ???? ????????????????????? ??? ????????? ?? ???? ?????? ??? ??????? ????? ????? ??? ? ?????? ???? ?????? ?? ??? ???? ????? ??????????? ???? ?????? ???? ?????? ????? ?????? ?? ????? ??????? ????????????? ?????????????????????????? ???????? ?????? ??? ???? ???? ??? ?????? ?? ??????? ?? ????? ? ???