The obvious advantage of a free-standing - movable - rack is being able to store/display watercraft on a portable, raised, multiple-cradled unit in a variety of outdoor locations. Racks with coasters provides an additional advantage when it comes to positioning boats to/throughout an area. Most free-standing racks are designed to accommodate at least two boats, others have the capability of extending their capacity to six boats. Some racks are designed for easy, one-person loading, too.
You also have the option of both brackets on the same frame or on two separate rack frames for custom/independent length support. If horizontal space is limited, consider storing your kayak vertically - there’s a rack for that, too!
Expansion and movability are two key capabilities of a mobile rack. Movement can put stress on both the brackets/rack as well as the frame, casters and associated hardware and accessories.
Besides storage, consider a free-standing rack for holding boats while doing repair/maintenance work or simply cleaning it. You can mount the boat at a convenient level and if on coasters, can move it into/out of the work area with ease.
A few other features to look for or remember:
A free-standing rack is quite the versatile storage unit. It’s works just like a wall mount but you don’t need a wall! It serves well as a mobile outdoor rack that can be moved as external influences dictate - or seasonally from a high-use/access area to a low-use/storing area at other times.
Also consider adding straps or other tie-down/security means, especially when loaded to the max’ (some units will support up to about 400 pounds). Conversely, know the limits of any rack, pushing the weight load might result in injury to you or others and/or damage to boats and other property.