If you keep people in prison for the whole of their lives you end up with at least one prison that is, in reality, a nursing home, a very expensive one.
A life sentence is that, even if you are released it is 'on licence' and you can be recalled.
If you are found guilty of murder in this country you can only be sentenced to life. One of the proposals that could be introduced to allow for sentences of 50 or 100 years, that way you would be eligible to be considered for parole at half way thorough your sentence.
But how can you tell relatives that their loved one is only worth 20 years? Or 30 or 50?
It puts a value on the deceased's life, it makes their death not as bad as another murder, or worse than another murder.
Example
At the moment there is someone serving 2 life terms for the Soham murders.
At the same time there is someone serving one life sentence for killing April Jones.
If you were a relative of any of those three little girls how would you feel if the killers had different sentences?