As someone who messes around with npcs/accessories/and meshes for certain objects in game. Accessing the toolbox is often annoying at times.
Having to keep two windows open, one for the catalog, and one of course for studio, just to grab/copy the id (using a certain browser extension helps 
) and paste it into the command bar with a line to insert it in the game/use a plugin. Or, worst yet, search in the models category to either find accessories/hats with a malicious script or messed up proportions.
Going back and forth constantly makes it a really time consuming process at times, especially when there are times when you need many accessories of a certain type.
Having the catalog in the toolbox will allow developers quicker access to meshes/textures Roblox created for avatars and or for decoration (ex, hat on a table). It also will allow developers to easily access certain things that were difficult beforehand to find/implement. For example, getting a player’s animation (ninja, cartoony, etc) will sometimes lead certain developers to hope and find that they get it from the toolbox (by searching in the models category).
TLDR, it would be very convenient for developers to have access to a wide selection of meshes/accessories made for avatars or decoration without having to go back and forth from website to studio.