When I first started my main, (who was a warrior), I started.off with skinning and enchanting. I did this because as a gear dependent class, I always needed money to repair my armor or make expensive purchases (everyone was poor back in BC, it took months to save up for swift riding skill). Being able to skin mobs allowed me to increase my loot yield. As much money as I made, however, there usually wasn't enough left over to get the next gear set a avilable in its entirety. so I chose enchanting as my secondary profession. Being able to make my own better without paying anything (almost) was a great help in my progression.
After hitting 70, I no longer was always short on cash and so my use for skinning came to an end. In its place I switched over to alchemy so I could make my toon through the use of potions, increasing my DPS and survivability. I also no longer was dependent on enchanting for keeping up with other people in terms of stats ~ I could pay other people to do it for do it for me. In its place I took on engineering because of 1. It's a fun profession and 2. It allowed me to have versatility that other warriors didn't have. I could res people with jumper cables, I could survive being knocked off the lumber mill in Arthur Basin by deploying my parachute, etc. In the my first pair of professions, needed to help me keep up with the difficulty of BC was no longer required and I could spend my time crafting more beneficial items. (Today, money flows out of the game content as if they were drops of water flowing from a river, professions are now choose mostly for transmog purposes or to get special items like gryocopters or the vial of the sands).