Beer Sheva's importance is its function as a central place for the entire Negev. Historically it developed because of the many wells, the most famous of it being "Abraham's Well".
Biblically, the site of Beersheba is where Abraham and his son Isaac made oaths of non-aggression with the Philistines, represented by a king named Abimelech. Abraham lived in the city for 26 years and his son Isaac lived there for many years as well. It is from there that Jacob set out on his journey to "Haran" the birthplace of his mother, to flee from his brother Esau.
Beersheba is also mentioned in Joshua 19:2. It was the southernmost city of Israel in Biblical times - hence the expression "from Dan to Beersheba" was sometimes used to describe the whole kingdom.