What are the main things consultants need to know when working with a business?

This actually depends on which is your question. Consultants won't do magic, they would just help you understand your business from a different perspective and give you advice based on past experiences and predictions of what they think it could happen.

E.g: Here in Clarity I got a question about how to promote a film deal. After talking to the person he told me he would have a premiere that was sold out already. I told him to extend the Q&A outside of the premiere and into a bar. To create a more close experience with his audience to obtain feedback of what they thought of his film (product). He should also obtain all the contact info of his audience. Once you do this, it's easier to address to them. 1-You know them better 2-You have their contact info
As you can see this is very specific. The same should be your request to a consultant

1. How to add massive value for their clients: Anytime a business is engaging with a consultant- they are expecting solutions to their problems. If you can solve their problems and add massive value to them, they will love you.

2. Communication is paramount: Businesses want to be kept in the loop. They don't want to have to reach out to you to ask you what you've been doing. Be proactive, send them status updates and treat them like you'd want to be treated.

In order to be hired as a consultant, you need to provide value to your customers with outstanding quality.

For instance, being a technical consultant means that you are more experienced than the technical department of your client, and they want to get a 10 out of 10 review on their platform, design the right architecture and coach the team in the best direction possible.

Business consultants can assist the CEO and the managers by finding out the best way for them to generate revenue, grow their user base or whatever the main problem of the company at that moment is.

Your end result has to be of a great significance to your client. You can focus on areas that require deep knowledge and a lot of experience - through trial or error, or working with other top corporation - which would justify the costs for you given the ROI.