I have a mid 2012 MacBook pro. What is the port immediately to the left of the thunderbolt port

ports available are:

Gigabit Ethernet, a single Firewire "800" port, two USB 3.0 ports, a "Thunderbolt" port that supports an external display at 2560x1600 as well as other peripherals that are compatible with the Thunderbolt standard, and an SDXC-capable SD card slot.

--according to everymac.com

See:

Mac mini (Late 2012): External ports and connectors - Apple Support

when USB-2 (pretty slow) was the only USB in town, it was not considered fast enough to connect an External disk drive, except a backup Drive where speed was not an issue.

FireWire-800 provides an inexpensive interface that runs at near-hard-drive speeds. it converts happily to FireWire-400 with some simple adapters or adapter-cables and was used on video cameras as a way to send standard digitized video from a camcorder into a computer.

SSDs can run faster than rotating drives, so more speed is needed to support them. Faster versions of other interfaces have made it less important.