Your site does not need to be perfectly flat to have a marquee.
The floor has lumps and dips.
If the lumps and dips are small, the matting will help smooth them out. When the lumps and dips are more pronounced, hard levelled flooring may be needed.
The floor is not flat.
A slight slope is not a problem for a marquee. As a rule of thumb, try putting a table on the slope, if the table is wobbly or cannot hold a glass you will probably need levelled flooring.
The floor is completely uneven.
If your site is very uneven you will need to hire levelled flooring.
The site is on a hill.
If your site is on a hill, you can hire suspended scaffold flooring.