It's about establishing and limiting legal liability. Being able to establish a solo business as a company means that you don't lose your house and possessions if your company fails.
Edit: in the US, that is.
For Canadians: https://www.ic.gc.ca/eic/site/cd-dgc.nsf/eng/cs06641.html
> Like shareholders of a corporation, all LLC owners are protected from personal liability for business debts and claims. This means that if the business itself can't pay a creditor—such as a supplier, a lender, or a landlord—the creditor cannot legally come after an LLC member's house, car, or other personal possessions.
https://www.nolo.com/legal-encyclopedia/llc-basics-30163.htm...
Obvious errors being breaking the law, paying for your business loans with personal funds, etc.