I get it for the character itself, you have to be level 50 to maximize gold, but what about the the crafting professions? Isn't it better if it stays level 1 so you can use easy and cheap materials (iron,jute etc)?
A lot (most ?) active in game characters are CP160+ so in fact refined rubedite etc are usually very very cheap as people refine it for the other mats and don't need the rubedite itself.
Also if you raise it you get better non-gold rewards - armour kits, chromium grains etc. The insane price of chromium platings definitely means it's worth maximising everything you can do in game to get them if you care about gold.
You get your crafting to 50, then you get your crafting skills to level 10, research all things, then you start to learn motifs; then finally during your daily writs you are more likely to get master writs. For alchemy and enchanting you are more likely to get 2-7 writs, for blacksmith, clothing and woodwork you get occasionally get 100.
Your writs can be used to get the 7 extra chests for storage, all the crafting stations for your home (I recall the basic ones cost 35), the craft station for armour sets, some very nice furniture, the training dummies and a couple of other things and the list keeps growing.
When you start to get master writs to create, for me at least, a whole new part of the game opened up! Just start the research sooner as you can, because near the end it will take you about a month to learn a skill. I've gotten a couple of 300 master writs to make and once a piece of 600 master writ jewellery , which took a lot of chromium to create.
Of course you can just beat up monsters instead, get a lot of money and in game buy most of what need instead, so its not required, but eventually it makes you life easier, for me it least.
The materials you find are 50% your character level and 50% your crafting level. Iron etc is only easy to find because you're either low level or you can only craft level 1. If you were max crafting level and cp 160 you'd only find rubedite making it even easier to find.
Its common to make one of your characters a master crafter. Crafting writs/dailies will give back purple and gold level mats as well as writ vouchers, exp, and food/drink recipes. Plus, when you decide to create another character, your master crafter can make them a set of training gear so they don't run around in mismatched trash gear. Finally, some of the events will incorporate daily crafting writs for which you can get very, very good rewards if you do it on the topped-out level.