This is done to make the language easier to say, and faster.
'I don't know' /I duno/ , /kamra/ for camera, and 'fish 'n' chips' are all examples of elision.
In the classroom
Elision is an important area in listening skills, as learners are often unable to hear elided words correctly, especially if they have little contact with native speakers. Word-counting and dictations are two activities that practise recognition, whilst at the production stage drilling elided forms such as contracted forms is common.