The value is in what it prevents. Without one, a designer produces a concept based on their interpretation of words like modern, clean, and professional, which mean different things to everybody. The feedback then arrives as this is not quite right, which contains no usable information, and the revision cycle begins. A mood board makes the target visible before the work starts, and disagreements surface while they cost nothing to resolve.
Build it from things you did not make. Photographs, colour combinations, packaging, signage, websites from entirely unrelated industries, typography you noticed. Unrelated industries matter, because collecting competitor work produces a brief that says make it like them, which is the opposite of the point. What you are identifying is a feeling and a level of formality rather than a set of features to copy.
Include what you are rejecting, which is the half most people omit and the more useful half. A section showing directions that are wrong, with a short note about why, communicates more than the positive examples do. Saying that something is too corporate, or too playful for the customers you serve, gives whoever is designing a boundary rather than only a target.
Annotate rather than only collecting. Under each image, one line saying what specifically you responded to: the restraint, the colour, the way the type is spaced, the confidence of it. Without that, somebody looking at your board sees fifteen images and has to guess which quality mattered, and they will guess wrong on several.
Do this even if you are designing it yourself, and particularly then. The discipline of assembling references and articulating why forces decisions you would otherwise make unconsciously halfway through, and it gives you something to check the result against when you are too close to it to judge.
Keep it after the design work is finished rather than discarding it. It becomes the reference for every subsequent decision about how the business looks, including ones made a year later by somebody else, and it explains the reasoning behind an identity in a way the finished logo cannot. When you eventually brief a photographer, a sign maker, or a printer, handing them the board communicates the intent faster than describing it, and it prevents each supplier interpreting your brand slightly differently.