Whether you have hook or tail on each end is a matter of preference, I have mine set up with snap shackles.
Having
one set up ready to go means it will save time in any emergency
situation.
The obvious use is to gain mechanical advantage when lifting heavy objects.
It can also be used to take the strain off another line.
With a rope tail on the block tied to a line with a rolling hitch, strain on the line can be eased in order to,
However, in practice there is a certain amount of friction generated at the sheaves.
Also the length of rope needed to move the load a certain distance will be increased by the number of falls. More information on blocks and tackles.