Steven
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Normally to reduce the starting current. Some Authorities have regulations on the size of motor at which you should start to use/consider star-delta. Technically as long as the starting current is not having any adverse effects on the electrical system you can run motors above 15 HP direct on line.

The site post ‘Introduction to Motor Starting’ (http://myelectrical.com/opinion/entryid/77/motor-starting-introduction) will give you some more idea.
Today I have also done a post on Star-Delta starting (http://myelectrical.com/opinion/entryid/83/motor-starting-star-delta).