It provides liquidity to business owners.

As a business owner, if you want cash today because you are done with a business. You could go to a bank and get a loan to pay dividends. This is a bad deal for the bank as you have no incentive to operate the business after you cash out the loan. A private equity firm comes in and operates the business on the model that they still keep some of the profits after the loan value.

The crappy side comes in as a customer, the PE firm can do this to an arbitrary number of firms in the area and raise prices on each/cut services. PE firms can trivially build out monopolies. Many of these monopolies will be invisible as they leave the existing branding etc. in place.