Put very briefly because I don't have time but it should provide you with a starting point.

If you're talking about HUMINT related intelligence then there are a couple of reasons:

The Germans generally had two ways to get agents into the UK- either by parachute or by ship from Spain, which of course was a neutral country then. Parachutists were easily enough detected and ship passengers could be searched and screened. The passengers generally had fake documents and could be detected relatively easily.

These agents were captured and were given two options: either be hanged as an enemy agent or work for the UK. This was called the Double Cross System. As I understand it, some were hanged just to keep the pretense up also. A necessity, unfortunately.

What this meant was that those turned agents could feed back disinformation to the Germans, thereby diverting resources from enterprises they could really have used them on.

After the agents had been "established" in place, the Germans would often send more agents to RV with them and create a network. Naturally the British knew about these ones and captured them too, thereby created a network of double agents.

I fully recommend The Double Cross System by Masterman. It's more or less a debriefing document from just after the war. There should be a copy online somewhere but otherwise you can get it from the usual places.

If you have any questions I'll be back in a few hours and I'll see what I can answer. I'm no expert but I am an interested amateur, ha.