Here's some reasons why..
Kurinji Flowers is set in 1930s India in a tea plantation in the Western Ghats. Staying in a 1930s tea plantation manager's bungalow was a perfect way to start to live and breathe the world and way of life of my characters. For example the sounds they wake up to each morning, the sights they would have seen, the smell of the tea fermenting in the factory, the colours of the landscape.
A sure fire way to lose credibility with your readers is to make factual errors. I've just returned from my 2 weeks in India and found several small things that can make a big difference. For example I had assumed that when the kurinji flowers (once every twelve years) it would only be visible up in the high grasslands - then one of the locals told me it was visible from the veranda of the bungalow. A tiny detail but one that makes a big difference to my character - it means she cannot avoid seeing it - and hence being reminded of what it means to her.
What are the plants and trees that grow there? The way people talk? How it feels when the rains come. What elephants' dung looks like (well maybe I won't be including that...). What a firefly looks like when it gets into your bedroom at night. What it's like to be woken at five thirty every morning by the siren wakening the tea workers. What wild honey hives look like.
I borrowed names from some of the people I met - so I know they'll be appropriate to the locale.
Just being in a place that's far from normal distractions and soaking in the world of your book helps you think about it in a different way. I came up with a completely new character who helps create some more dramatic tension. I invented two completely new scenes that give the key players more scope to develop. I also cut out huge chunks - happily killing my darlings.
I am a great believer in writing retreats anyway - I have been three times to Retreats for You down in Devon in order to write. Dispacement from your usual environment can have an electric effect in inspiration and output. One of the big pluses of a writer's retreat is not having to worry about day to day life - and particulary cooking, shoppping etc. My trip to India was equally restful - as I was waited on hand and foot (I was the only guest in this fabulous place). The only downside (which was a plus for my liver) was the lack of alcohol as Kerala is a dry state - whereas it flows freely down in deepest Devon!