Europe has many countries with many different legal systems so you can't make such blanket statements. Especially depending on how you define Europe.
>I don't even know what's the point of such sentences. Is it for PR purposes?
It is not for PR purposes, usually.
It is so that if a person is acquitted of one of the crimes they still stay in jail for the others. If someone gets 50 years for rape and murder and is later acquitted for murder, but not rape, they still serve their rape sentence.
The judge can decide if the jail time is served concurrently or consecutively.
It seems silly to give someone 3 life sentences, after all, you only have one life. But that means they'd have to be acquitted of 3 different crimes to get out of jail. The other reason is if those life sentences come with the possibility of parole after x years - by stacking those life sentences consecutively it would delay the parole hearings until after the person is dead.
Buy sometimes it is used to emphasize the crime's severity. Such is probably the case with the 40,000 year sentence for the Spanish terrorists. Even though they will only serve 40 years - the max allowed by Spanish law.