Well, it’s hard to explain why - more a feeling but I’ll try.
They start by collecting you from the station all together and that’s your ‘team’ and then after the security stuff (handing in phone etc) you go to a room and have a production team with you all the time.
The time is basically spent getting the team to gel, making you relaxed, feeding you etc.
You play some fake goes of the game to make sure you understand the rules. They get you excited about which chaser it might be (genuinely a surprise) and the the wardrobe lady comes to look at the clothes you brought and they decide what you should wear.
After a couple of hours of all that you get some time alone in a dressing room. You might see some famous people (I saw Russell Kane down a corridor).
Then you go to the studio. The floor manager was lovely and we got to laugh a lot during filming between takes.
I got very excited by the buzzer and it took several goes before I stopped squealing each time I pressed it.
Honestly, written down it sounds really pedestrian but it was masses of fun.