Once you put OSRS gold the cash inside it turns into a Guild Coin. One Guild Coin is the same as one ordinary Gold section. You can only use the currency for those guilds needs. That's why when you contribute you can be confident that the owner won't spend the money on some thing else such as armor, food or anything he can use for himself. All your cash will be unable to help the guild. Here is what can you do with your Guild Coins:
Pay/Buy Servant - If you have sufficient Guild Coins you can pay your winner for his services or purchase one from Ardounge.
Buy stuff - You can buy materials (planks, nails, gold leaves etc.) to your guild with the coins. But only from NPC stores. Guild Coins are untradable so that you can't exchange them to any player and you can not purchase the materials. These materials with ( c ) after their name are untradable and may only be used on your guild. That means you can't rely on them in your typical POH.
You can Find a Dead Tree seed out of Wyson the Gardner in Falador for 500,000gp or from any Revenant drop. You require level 35 Hunting to catch them and you're able to catch them at the Tree Gnome Stronghold swamp with no tools. If you don't have the required Crafting level then deliver the stuff to Master Crafter in the crafting guild. He will craft it for you for a fee.
When I found out there was a Cooking Guild, I wanted to get indoors right away. It took 32 Cooking, so I got there immediately! I went inside just to view, grapes, a pot, pan, a chocolate bar, apples, and some other things that have NO usage. I believe that Jagex has the capability to make a far better Guild than that! Additionally, the Ranging Guild. The only things you can do there are hit targets and purchase things that may be purchased in Varrock already! Is that a joke? I'm SUGGESTING that these guilds can be redone buy RuneScape gold to be produced better like the Warriors Guild.