There are two main reasons. The first is that they feared them for the same reason they feared submarine warfare. Britain remains an island nation, dependent upon regular supplies of food and materiel. Successful surface raiders, like submarines, can cut seaborne supply lanes either by destroying shipping or forcing ships to stay in port.

The second reason is related to what's known as a fleet in being. Often, the threat of military action forces you to do things you wouldn't otherwise do. If you know your enemy has a large force or a weapon that it has not committed to battle, you must take precautions of your own for the moment when that force or weapon does enter the battlefield.

When it came to surface raiders, the Royal Navy was forced to divert ships, sailors and aircraft to contain and destroy them. This diluted the Home Fleet and reduced the reserve of force that Britain had to deal with an unexpected German move.

/u/The_Alaskan gave some really good information. Just to add that it wasn't only ships coming to the UK that were at risk. After the Tirpitz was located in Norway the prime concern was that she'd attack the convoys leaving the UK that were going to Russia.