PSR 275 MIDI--connecting to my PC

I just bought a used PSR 275 and I am a complete newbie. I bought it to have come percussion accompanyment for my banjo. However, my new keyboard is really cool and appears to have many capabilities.

Anyway, it came with a MIDI cable but the PC end of the cable has a large, flat plug, not a USB plug. I can't find a port on the back of my PC that will accomodate it.

Do I need to get a different cable with a USB plug, or do I need to get a new board for my PC that will accomodate my plug?

Once I get it connected, do I need special MIDI software? Where can I get the necessary software?

Finally, what benefits would I get from connectin it to my PC? (i.e. why would I want to do this?)

Thank you for your help,
Eric