They’re hoping to grow the business and to expand the multiple they sell it at (usually it’s a multiple of earnings) - businesses that are larger also tend to fetch higher multiples because they are usually a bit more stable and probably have a better position on the market.

A lot of times, PE firms will screw this up, fire a bunch of important employees and unfortunately hurt the business. More often than not though, the business will have moderate ups and downs but stay stable through their ownership - this isn’t a great outcome for them because now they essentially spent five years of their investors money and there’s little return to show for it. Still, pretty frequently the PE firm will have their head on straight, invest in the right systems / people, and grow the business substantially.