DB9 connectors were once very common on PCs and servers. DB9 connectors are designed to work with the EIA/TIA 232 serial interface standard, which determined the function of all nine pins as a standard, so that multiple companies could design them into their products. DB9 connectors were commonly used for serial peripheral devices like keyboards, mice, joysticks, etc. Also they are used on DB9 cable assemblies for data connectivity.
Today, the DB9 has mostly been replaced by more modern interfaces such as USB, PS/2, Firewire, and others. However, there are still many legacy devices that use the DB9 interface for serial communication.