Rafina is the smaller of Athens' two main ferry ports, on the east coast of Attica, and for many people leaving Paros it is the smarter place to land. It sits closer to Athens International Airport than Piraeus does, so if your trip ends with a flight home, a boat to Rafina can cut out the drive across the city. The port area has cafés and restaurants, so a gap before an onward bus or flight is easy to fill.
The route mostly runs in the warmer half of the year: Ferryhopper lists it from April to October, with boats every day in that season. Three companies share it. Seajets runs high-speed boats and is the fastest at around 3 hr 15 min to 3 hr 30 min. Golden Star Ferries operates a mix of ships and takes about 3 hr 45 min to 3 hr 55 min in this direction. Fast Ferries runs a conventional ship that takes around 4 hr 10 min and carries cars.
Not every sailing is direct: some call at other Cycladic islands on the way, and each stop adds time. That explains why Ferryhopper's overall range stretches up to 6 hr 45 min on the slowest itineraries, and why it is worth reading the stated duration of each sailing before you choose.
Before booking, match the arrival time to your plans on the mainland. Ferryhopper says departures in peak summer usually begin around 08:15 and finish around 16:35, so evening arrivals in Rafina are common. That suits a late flight or a night in Athens, but it can be tight for an evening departure from the airport, so leave a generous buffer.