All games on PC platforms are riddled with occasional bugs and glitches on their first release and even a top selling game like World of WarCraft is no exception. Patches are not only meant to fix mistakes on the developers side, but they can also include some minor tweaks to the game play, add some new enhancements, or plug in exploits that other players take advantage of. Having the latest official World of WarCraft patch will allow you to enjoy the most stable build with all the latest fixes and enhancements that the developers applied for all online players.
Since World of WarCraft is purely an online RPG, you cannot play the game without installing the latest World of WarCraft patch in your system. This is because Blizzard checks your version when you log in to the server and compares it with the server version if it is equal or not. This is to ensure all players are equally balanced and share fixes and feature set.
A World of WarCraft patch is around 20-40 MB and can escalate as future patches are released. That's a big file size for a patch and usually has a long list of changes making these patches worth the wait for downloading and installing.
Patches are conveniently downloadable from the Blizzard servers on demand while the game is running so you don't have to worry about finding the patches yourself. However there are plenty of mirrors around that offer several links to the latest patch. Try to avoid these whenever possible as they may contain viruses or trojans that can modify your system and possibly corrupt your game.
Even though World of WarCraft downloads the latest patch when needed when the game runs, that shouldn't stop you from checking out what the patch does on Blizzard's Patch Notes section. Many of the changes usually consist of exploit fixing and some improvements to the features. This can affect both your game play and strategies whenever a patch is released. By knowing all the major changes, you can adjust ahead of players that don't bother checking it out.
Any World of WarCraft and expansion owners should really check out notes regarding previous patches while the others should upgrade to the latest World of WarCraft patch as soon as possible in order to play. The updating process is usually hassle free to most gamers and after it's finished installing; all you can do is enjoy the features while you can until a next patch is needed.