"Why Camels?"
After the Mexican-American War of 1848, America was asking this question, because it suddenly had to deal with a huge amount of desert in its new southwest territory. A spate of books came out on camels. I have four, including one by an importer for zoos and circuses, and one for Congress by a team led by Jefferson Davis, future president of the Confederacy.
Basically, your horse is a steppe creature, built for speed on hard dry earth, eating grass, and surviving cold winters. Your dromedary/Arabian/one-hump camel is a desert creature, built for moving on soft sand as well as desert pavement, eating anything including thorny acacia, and surviving without much water.
So which you use as a war mount depends on what you have available to you and which suits the job. If you don't have 'em, you can't use 'em.
It also depends on if your mount has been inured to dry country. Camels that travel from village to village along the coast of Africa don't last any longer than a horse without water. The ones on the long-range desert caravans have gotten used to it.
One thing to note: the smell of camels spooks horses not used to them (they're pretty disgusting for the humans, too). This has been used to disrupt cavalry. It also resulted in closing down all 3 attempts at a US Camel Corps, in California, Nevada, and Texas, because you couldn't take them anywhere that they didn't run into horses that fled them. The last was in the 1870s, but there haven't been reports of feral camels since the 1930s. Considering that camels aren't natural herders (they don't stick together in caravans unless tied to each other), I'm surprised they lasted that long.
SOURCES:
California Legislature. Senate. Committee on Internal Improvements; Report of the Committee on Internal Improvements, on the Use of Camels on the Plains; 1855; [Sacramento] B. B. Redding, State Printer; http://www.archive.org/details/mmittereportofco00calirich Croal, Thomas Allan; A Book about Travelling, Past and Present; 1877; London and Edinburgh, William P. Nimmo; http://www.archive.org/details/bookabouttravell00croarich Daly, H. W.; Manual of Pack Transportation; Washington, DC, War Dept., Office of the Quartermaster General,circa 1910; http://www.archive.org/details/ Davis, Jefferson; United States. War Dept; Report of the Secretary of War, Communicating in Compliance with the Resolution of the Senate of February 2, 1857, Information Respecting the Purchase of Camels for the Purpose of Military Transportation; 1857; Washington; A. O. P. Nicholson, printer; 1857 http://www.archive.org/details/reportofsecretar00uniteds Edward, Lord Gleichen; With the Camel Corps up the Nile; 1888; London, Chapman & Hall, limited; http://www.archive.org/details/withcamelcorpsup00gleirich Engelbert, Victor; "I Joined a Sahara Salt Caravan"; National Geographic, November 1965 Fabens, Joseph Warren; The Uses of the Camel: Considered with a View to His Introduction into Our Western States and Territories. A Paper Read before the American Geographical and Statistical Society, March 2d 1865; 1865; New York; Carleton; American Geographical and Statistical Society; http://www.archive.org/details/usesofcamelconsi00faberich Farrow, Edward Samuel; Mountain Scouting: A Hand-book for Officers and Soldiers on the Frontiers: Profusely Illustrated and Containing Numerous Notes on the Art of Travel; 1881; New York; E.S. Farrow; http://www.archive.org/details/cihm_14973 Leonard, Arthur Glyn; The Camel, Its Uses and Management; 1895; London, New York, Longmans, Green, and co.; http://www.archive.org/details/camelitsusesmana00leonrich Marsh, George Perkins; The Camel; His Organization, Habits and Uses; 1856; Boston, Gould and Lincoln; http://www.archive.org/details/camelhisorganiza00mars Silver, Caroline; Guide to the Horses of the World; 1975, 1990; Elsevier Phaidon, London; Chartwell Books, Secaucus, NJ Steel, John Henry; A Manual of the Diseases of the Camel and of His Management and Uses; 1890; Madras, Lawrence Asylum Press; http://www.archive.org/details/manualofdiseases00steerich Van Ess, John; "40 Years Among the Arabs"; National Geographic, September 1942 Young, Henry Pottinger; Hints on Horses; with Short Notes on Camels and Pack Animals; Also a Few Practical Suggetions on the Training of Polo Ponies and Players, and Gymkhana Training and Racing; 1907; BryansÂford; http://www.archive.org/details/hintsonhorseswit00youn