Another thing to consider is the “Cost” of the tiers. The game started with only up to exotics, and thus are considered the baseline for the majority of late game activities. It should also be noted that leveling in this game is designed to be extremely fast, given it intended WvW and sPvP to be the primary late game; where as the PvE side was meant to be more casual play, or something to do between major PvP/WvW seasons.
So with that in mind, gear while leveling up tends to be largely inconsequential. So much so, that you’ll be leveling faster then level appropriate gear will drop for you. Builds also tend to be unimportant for most of the leveling process… though some classes are more build dependent then others.
Once you hit lvl 80, you start entering the buildcraft portion of the game, and decide what game modes you take on, and what role you want to play in them. End game content is very group centered (Raids, WvW, Map Meta chains), while exploration and Core Tyria events are played very loosely, and can be done solo for the most part. Group content is really the only parts of the game where Team composition matters. For everything else, a build with a bias toward damage, minor self-support and mobility is all thats really needed to avoid dying.
Again, Exotics are the game’s base line. Ascended gear was added (initially) to support infusion slots for Fractal mini-dungeons. Fractals use a unique environmental hazard known as “Agony”, which is a purely gear check mechanism used to gate access to higher level fractals. You would build an ascended set over time, and incorporate infusions (which you gather through play) to raise agony resistance. The catch though is the Ascended armor set very expensive to construct…. a whole order of magnitude more expensive (currently 300g if you buy everything). The main reason for this was to force investment for high tier fractals (which have significantly higher pay out), and served to help drain the Trade Post of excess mid tier materials. This secondary purpose is really the reason why the construction cost is so high right now. As time went on, they added additional methods to get ascended gear as a way to undermined the reoccurring cost of gearing multiple characters… but you had to build a set initially to get into the new reward loop. It didn’t make the process cheaper… but was less complicated then crafting. It wasn’t until a year after that they increased the stat value of Ascended gear as a response to players being upset over the steep price increase between it and exotic tiers. (And at this point, exotics could be had for free, by doing several dungeon runs over the course of a few days). Today, if you have access to certain LS3 maps, and are a regular in raids, you can get ascended gear without needing to craft anything.
Legendary gear started in a similar situation. Legendary weapons were expensive to construct, and were considered the ultimate end-game crafting project. But these items were purely cosmetic, and meant to be transmuted/merged with another weapon with the desired stats. I forgot the exact time line- but around the time Asc gear got its stat upgrade and the Wardrobe system was introduced, they added a new mechanic to Legendary weapons that allowed them to infinitely change their stats, and could choose from any stat combo available in the game. It was a gesture to reflect the high cost of construction and give legendary items some form of utility, since you couldn’t use transmutation to change the stats on it anymore. Legendary armor was a recent addition (something they previously refused to do), and tied it to Raid rewards. That utility is also something that grew out of the demands of raids, since classes often need to change stat combos in order to change roles within the Raid meta.
So to recap…. Exotic gear covers 90% of all activity in the game. Ascended gear is only required for Fractals, due to them being the first tier to include Infusion slots. Asc gear also represents a massive price jump due to its role as gate keeper for high level fractals. Legendary armor is long term reward from Raids, and is mainly a gold/time sink trade for convenient stat swapping. Legendary weapons are for prestige/cosmetics, but does include conveniences in light of its high cost to obtain.
