Sometimes I want to add a command to a custom toolbar.
I find it frustrating to open nearly all the toolbars to find if there is a ready made button for the command I want. Obviously, one guesses as close as possible to the correct toolbar
Also, if you open the workspace editor - Options>Toolbars, then Tools menu > Workspace editor - you will see something that looks like the image below. In the lower box are all the toolbars, if you click on one, the upper box will show what the toolbar contains. Mousing over the button in the upper box will show the tooltip; clicking on the button will load the button into the editor window at the right.