Along with the kickboard, the pull buoy is one of the most common pieces of equipment swimmers roll with. There are downsides to using them, but they also have benefits if they are used with focus during swim workouts.
There is an obvious reason why most swimmers cling to their pull buoys when they step out on deck: their use makes swimming much easier. With added buoyancy comes better alignment in the water, and without having to kick to keep your hips high, you can conserve energy that would otherwise be guzzled by your legs.
As a result, for far too many swimmers the pull buoy is a crutch, and not always the training aid it is supposed to be. When it is leaned on too much, or used incorrectly, it simply covers up the flaws in your swimming stroke, keeps your hips largely flat (bad news for hip-driven freestylers and backstrokers), and ends up doing your core's job in providing balance and stability for your stroke.
That being said, when used in moderation and tactically, the pull buoy can provide some improvement for swimmers.
You can overload your arms
Strictly using a pull buoy might not accomplish this (particularly for elite-level swimmers), but doing pulls with swim paddles and a band will, especially if you add a band (a black rubber strap that keeps your ankles together). Because your legs are tied up—literally—you can really hammer away at your shoulders, back and arms.
Master your breathing habits
One of the most widespread training habits among swimmers is lack of attention to their breathing pattern. They simply don't think about it. As a result, they breathe every stroke in and out of the walls during harder sets, and they get winded sooner. Without your oxygen-hungry legs sapping you, pull sets present a perfect opportunity to work on bilateral breathing, breathing every 3-plus strokes, and not breathing in and out of the walls.
Teaches you better body position
The secret to faster swimming isn't really a secret—it's to present as slim a profile as possible in the water in order to glide through it with maximum efficiency. When a swimmer's hips drop, it creates a much larger drag profile compared to when their hips stay nice and high. That's why swimmers with terrible flutter kicks and bad hip positioning can manage to pull nearly as fast as they can swim.
It can help with your feel for the water
One under-appreciated aspect of doing long, boring pull sets during swimming workouts is that if you do them with attention to technique and distance per stroke, your feel for the water will improve. There will always be debate about quality vs. quantity in prescribing sets and reps for practice, but there's no getting around the necessity of getting meters and yardage in to help hone and develop your feel for the water.