It always depends on the needs of the business. Two reasons I've seen business that had access to PE or Bank financing take PE financing:
1. Team expertise - Some PE firms have a staff of ex-McKinsey / Bain / BCG ready to jump in as VPs / CSuite and next level the business. The people that got you from A to B might be highly competent crushers but lack the skills to go from B to C.
2. Liquidity event - Some founders want to both grow their business and cash out some of their sweat equity today to compensate for deferred purchases that were invested into the business (ie a house, shored of retirement savings, a 50-foot yacht) . Hard to cash out your equity with a bank loan.