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In World of Warcraft, there are many various trades to learn, all capable of earning money and creating useful items for your character. Alongside the profession are secondary skills, skills that anyone can learn and use -- those being first aid, cooking, and fishing.
First aid and cooking seem straightforward enough; they are capable of healing and giving buffs to players respectively, skills that are great for those whole can't heal and saving mana to those who can. That and unlike the main trades they are cheaper to learn and easier to master, usually requiring only doing a repeated action like creating bandages or food over and over again with a small quest chain in later levels.
But fishing? For a long while it was merely a side quest, an activity for those who liked to simply fish up random treasures or to participate in the Fishing Tourney every Sunday. But after the Burning Crusade expansion, fishing received a serious boost in usefulness.
Fishing is directly useful to cooking.
Many cooking recipes, especially the later ones introduced in the second expansion onward, gave buffs to players alongside the direct benefits of slowly healing. In Wrath of the Lich King, the areas where named and cook-able fish were plenty giving players and easy way to fish up and cook stacks of for for future raids.
Food can boost any number of your players stats to maximize damage or healing during raids, questing or even player versus player. Fishing alongside leveling your cooking skill is also highly advisable as most fish are also used in cooking recipes.
Learning fishing opens up dailies later on.
When the daily quest system was introduced, being at a certain level of fishing was required for each one. When a player is at high enough fishing skill, the daily quests become available to complete, gaining you a bunch of gold and the chance to gain even more gold in the quest reward satchel (filled with either junk, or rare items, all selling for a decent amount at the vendor).
Fish sell decently on the AH as they are used for various trades.
Some players just do not wish to farm for fish; they may just want to skip the time to fish and to buy up a stack of what they need on the Auction House. Fishing up key fish such as those used for raid consumables, cooking achievements, or strange buffs (like the pirate/ninja buff) sell for a decent amount of gold. They are paying for your patience and farming skills.
Learning Fishing is the easiest (but most tedious) of the trades.
Fishing is leveled by, well fishing. Areas had certain levels where you could skill up your fishing, having you be able to fish in progressively more higher leveled areas. This was fun but also dangerous due to mobs (and other players if on a pvp server).
But overtime it was changed so that fishing in any body of water will give you skill ups, just that it was faster the higher level the area. I.E. you could literally fish in wailing caverns till your fishing skill was maxed. That being said, fishing is a time consuming and very slow going. I suggest popping on some music and fishing a few levels each day. You can just sit and level all day but breaking up the tedium can make the process more bearable.
Fishing makes you eligible for the fishing tourney.
The fishing tourney takes place every Wednesday and Sunday. The contest is simple; either in Northrend (Wednesday) or in Booty Bay (Sunday, pre Cataclysm expansion) you are to pick up a quest where you are to fish up a specific fish over the course of a specific time. The catch is you need to compete with the whole server in fishing up the fish and turning it in first. The rewards of bragging rights and fun mementos make this world event worth doing at least once.