Tin is useful because it can be alloyed with copper to create bronze - typically 88% copper (Cu) and 12% tin (Sn) although ancient bronze pieces can vary in composition. The word is of uncertain origin but this metal alloy was historically important enough that we have a period of human history called the Bronze Age.
The earliest period of Homo sapiens history is called the Lithic or Stone Age (subdivided into the Paleolithic, Mesolithic, and Neolithic). During this time, humans made tools and weapons from stone - often flaked quartzite, chert (flint), or obsidian which chip into nice sharp edges.
One of the first metals to be used for tools and weapons was copper. Native copper (pure copper metal deposits) occur in a number of areas and copper is relatively soft and easy to work. The use of copper appears earliest in Asia and appears to quickly spread into Europe. In Serbia, a copper axe was found dating to 5300 BC. Ötzi the Iceman, found frozen in the Alps in 1991, had a copper axe and was dated to 3300 BC. Copper working independently arose in the New World appearing in the upper Midwest (from copper deposits in Michigan and Minnesota) and among the Inca in Peru.
Sumerian copper dagger (c. 2500 BC)
The period of time when humans made copper tools is sometimes referred to as the Chalcolithic Age (from the Greek χαλκός for copper). Relatively quickly, somewhere in the Fertile Crescent, it was discovered that if a little tin was added to copper, it made a much harder metal. This occurred around 3500 BC. This was the result of intentional experimentation since copper and tin don't normally occur together as mineral ores. Tin has to be mined and smelted separately (from the mineral cassiterite), and then added to copper to obtain bronze.
Bronze weapons from Canaan (c. 2000 BC)
Copper is available throughout Europe and Asia, but tin is a bit less common. Starting in the early Bronze Age, tin was mined from the Cornwall and Devon areas of southwest England and was eventually traded throughout Europe. Much of the inter-cultural communication throughout Europe and Asia in ancient times was a result of mineral resource trading.