In piano or organ music brackets are used to show that some notes in one staff are to be played by “the other” hand. Or they indicate that dynamics or text belong to the notes within the bracket.
You also now have the ability to create custom symbols. I used a unicode character to add the LH bracket in one of my scores and it worked reasonably well. Not quite perfect, but perfectly acceptable.
You can use Engrave > Playing Techniques to create custom symbols. You’ll find the necessary brackets in the ‘Keyboard techniques’ range shown in the editor.