Because doing so makes the people of Singapore better off.
Imports of capital goods to create value-added in the country by using abundant human capital.. It will make Singapore richer and faster. Don't forget that Singapore has to import drinking water as well.
I lived in Singapore for a month, so my answer for you is a reflection of that experience and what I learned there (briefly) from talking with some co-workers. Singapore is a wonderful little country that is in between several economic powers - Malaysia, Indonesia (the 5th largest country in the world), China, Japan and the Koreas; (but you know all that already) - It is a very favorably positioned and accessible seaport. These trading policies were undoubtedly part of the policies of P.M. Lee Kuan Yew. It is a very active, willing and cooperative trade partner and in return gets great deals on imported goods from sea trade in return for favorable trading terms. I personally would love to live there, but the heat and humidity are too much for me to make that decision. HTH
naturally, to export it ( mostly) after awhile ...trade hub as it is.
