What exactly is the point of them? Isn't it essentially just saying 'ok you're going to serve 5 years for this crime and absolutely nothing for this one but if anyone asks, just say you're serving them both at the same time'.
Pretty much, it's like you get sentenced for six crimes but you only serve time for one. Imagine if they applied the same logic to parking fines.
The relevant principles are those that require judges or magistrates to take into account the 'totality' of the offending. You can download the definitive guideline from the Sentencing Council website ('TIC's and Totality') under 'Guidelines to Download'. That explains the relevant principles.
That would be pretty cool actually! This is effectively what happens if you want to park for the weekend in Helsinki. The average car park is in excess of 30 euros a night. A parking fine is 45. So, our usual routine is - arrive Friday evening, park on side street next to hotel, get up monday morning, pay fine, leave. Obviously it only works if you're not planning on driving anywhere during the weekend!
If a person is arrested, for burglary for example, the police will ask him/her to own up to other burglaries in the area even if he/she didn't commit them. This is known as "Taken into consideration" (T.I.C's) and the person will receive a short sentence for the original burglary for owning up to the T.I.C's, for these he/she will receive a concurrent sentence for each burglary. More importantly for the police it clears up their burglary rates even though the original burglar/s are still out thieving, basically it's more police corruption.