Sorry, where's the code you're referring to specifically?

There have been lots of changes over the years to port between GTK versions and UI redesigns, so the original reasons might no longer be relevant.

Pinta.Core/Managers/ToolManager.cs.
I removed this code and didn't notice any ill effect:

	private void HandleToolBoxToolItemClicked (object? sender, EventArgs e)
	{
		if (sender is ToolBoxButton tb)
			SetCurrentTool (tb.Tool);
	}

Probably needs more testing , but yeah I'm not sure if it's necessary anymore - SetCurrentTool() doesn't do anything if it's already the active tool.