If you mean this weapon:
it's probably a rich man's show-off toy and/or for hunting. Having a crossbow adding weight to your sword makes your sword rather less usable as a sword, and having a sword on your crossbow makes it harder to load. Neither of these makes it desirable as a military weapon. However, for hunting, where you can span your crossbow beforehand, at your leisure, and your prey will be long gone before you reload anyway, it is still quite functional as a crossbow. As a sword to stop an attacking wounded animal, or to kill a wounded animal, it would be quite functional. Indeed, the crossbow would act to keep an impaled charging wounded animal at a safe distance from the wielder, in the same way as toggles and crossbars on boar spears and boar swords.
Combination weapons including crossbows are relatively rare compared to those that include firearms. Most combination crossbow weapons are combination crossbow-firearms, either predominantly crossbow with an added pistol barrel, or a gun with an added bow:
These crossbow-guns are usually hunting weapons.
Combination weapons have been used on the battlefield, but apart from the widely-used and successful gun + bayonet, were relatively rare and often unpopular. Some example include the Elgin Cutlass Pistol:
used by the US Navy in small numbers from 1838 through to the Civil War and the Swedish Model 1703 axe-carbine:
used by the Swedish navy into the 19th century. Both of these weapons were intended for naval boarding parties, where weapons shorter than a musket could be useful in cramped quarters on a ship. For general battlefield use, combination weapons such as these were inferior to a musket with bayonet, which at least had reach to make up for its weight (a musket with bayonet is much heavier than a typical spear). Combination military weapons were still being made in the 20th century:
but apart from the still-present gun + bayonet and combinations such as assault-rifle + grenade launcher, have failed to see widespread military use.
One unusual combination weapon is this triple weapon, a gun-warhammer with a double-spike bayonet:
Triple weapons like this are rare, because the weight and clumsiness problem gets worse with each added weapon.
Apart from hunting and military use, some combination weapons were favoured by criminals. One famous example of such is the Apache pistol, a combination revolver-brass knuckles with mini-bayonet:
https://commons.wikimedia.org/wiki/File:Apache_revolver.jpg (a folding version)
https://commons.wikimedia.org/wiki/File:Deleaxhe_Apache_pistol_7mm.jpg
named after Parisian criminal gangs called "Apaches". While a triple weapon, it's small and compact. With basically no barrel, it's a poor firearm, but it wasn't intended for serious gun battles - a poor gun can suffice for muggings.
Finally, another non-gunpowder combination weapon: a Japanese yumi-yari, a combination bow and spear:
