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Ubuntu has traditionally had many different tools for application installation and management: the Synaptic Package Manager, Add/Remove Programs, the Update Manager, Software Sources… not to mention the command-line tools dpkg and apt. Those were related tools for doing similar tasks around software management. Before undertaking any installation task, you first needed to know which tool to use.
And many developers weren’t even coordinated to the extent of making their applications available from a single source. So an effort was to be made in two directions: the making of a single, all-encompassing tool, and the publication of all available software in that tool. Thus the Ubuntu Software Center was born.