Watching the detectives

We all know detectives come in pairs. In the movies, mismatched with a younger more flamboyant colleague. In books, usually paired off with someone who has a great admiration and understanding of them. In reality, of course, detectives work with a very large team and not a partner, so why do our fictional detectives need a sidekick?
Now that I'm working with my own pair of fictional detectives in my latest novel, I can tell you the answer! My detectives aren't the central characters, so we're never going to see them at home or on their day off. Without a sidekick, my detective would interact only with his suspects, which provides me with a limited view into his personality. As soon as he can walk out of earshot of his suspects and engage with his partner, so my readers can engage with him.
His partner isn't there for light relief, as can be the story with authors such as Agatha Christie or Conan Doyle, nor as a foil. But together their asides and banter breathe life into them that suspects alone can never achieve.
For example:

‘What’s first on the agenda?’
‘Interviews.’ Detective Chief Inspector de Freitas slipped his notebook into his coat pocket. ‘I love interviews, don’t you, Will?’
‘Sifting out the straight edges of the jigsaw to start building a picture.’
‘You’re one of those, are you? The wife always starts with a feature.’
Detective Sergeant McCulloch shook his head. ‘Couldn’t work like that. Corners first, then the edges.’
‘So who’s our first corner, Will?’
The sergeant flicked open his notebook and came to a quick choice. ‘Anita Wallis. Top left.’
‘To the bat mobile, then.’

The partner's presence might not be accurate according to police procedure, but partners are an integral part of the fictional detective's world and I for one couldn't do without them!