They have a ring-shaped fusion reactor, like a Tokamak, inside the ship
The crew quarters is smaller than the reactor. It is necessary to make the ship circular, so that the donut-shaped reactor fits around the edge.
Or some other large, circular piece of alien tech powering the ship and requiring it to be that shape. The Tokamak is ring-shaped because that's a good shape to magnetically contain plasma. Perhaps the UFO's reactor is ring-shaped because they are magnetically confining something else, such as antimatter.
The ship needs to spin to provide gravity
This might not be an issue on Earth (or it might; maybe their native planet had gravity like Jupiter) but in space they want gravity so they need a ring-shaped ship that can spin. The bulging center of the ship is where the drive is, with living quarters around the perimeter.
They find rotational symmetry aesthetically pleasing
Just a question of style. They like the saucer shape the same way humans in the 60s liked fins on their cars.
They need rotational symmetry so their warp drive calculations aren't too complicated
They need to warp space into a specific shape, conforming to the ship, in order to travel long distances. If the ship is symmetric, then they can warp space in a symmetric way too, which is computationally easier/less energy-intensive.
Their ship is capable of changing direction very rapidly and abruptly at high speeds, as UFOs have been observed to do, but air resistance is an issue. This rapid maneuvering capability is necessary because the UFO is built as a dogfighter against other UFOs. A saucer shape lets the UFO change direction very rapidly, without needing to rotate the ship first, while being aerodynamic in any direction.
The tractor beam works like a tornado
UFOs are often depicted as projecting a beam below them, which levitates unwary cows or farmers up into the ship. One thing on Earth that is column-shaped and can pull people straight up, is a tornado. Perhaps the UFO or some large component of it needs to rotate very fast as well, whirling some mysterious ambient energy into an "energy tornado" that can pull up passengers/test subjects.
Rockets spin for stability. Perhaps on an interstellar trip, especially in warp drive, it's very important to thrust straight backwards, so you don't drift even a little off target. If the disk spins, that ensures that the exhaust coming out the bottom of it is aimed straight back.