You need it so that the chain will not jam in the engine at high speed, which is a very dangerous situation.I agree with mralaska. I see no need for the tensioner
Removing one link of chain shortens it by one inch. One inch of slack in the chain can be fully absorbed by 1/2 inch of adjustment because every inch you move the tire from the engine is twice that on the chain. A total of 3/8 inch adjustment is all you will ever need to keep a chain properly adjusted. Some bicycles, as noted above, are able to adjust enough by moving the tire. I found it easier to move the engine away from the tire with shims because the tire kept going crooked when I tried to adjust it at the tire.You need it so that the chain will not jam in the engine at high speed, which is a very dangerous situation.
This is unless you are lucky enough that when you shortened the chain to the maximum allowable, it is adequately tight already and the chain wont stretch from then on (unlikely).
Mine did, alot. It stretched enough to pose a danger, but not enough for me to be able to shorten one more link. Hence the need for tensioner.