Advantages- usually more power due to straighter intake runners(no pushrod ports to obstruct -shape/airflow) and higher rpm limit due to lighter valvetrain, easier to run multiple valves for more...
well in diamond, pearl and platinum you go to jubilife city after you beat the third gym. go inside the poketch building. on the first floor talk to the president of the company and he'll give it to...
After you catch the main legendary (dialga, palkia, giratina) go to lake verity and mesprit will be in the cave in the middle of the lake. Talk to it and then it will flee and appear on your map app.
You get the Marking Map from the Poketch Company President after getting the Cobble Badge (gym in Veilstone City).
Sources: www.bulbapedia.bulbagarden.net and www.pokemon.marriland.com