DoorDash just made an offer to buy the United Kingdom-based food delivery company Deliveroo for $3.6 billion. If the deal goes through, it would get DoorDash — which does app-based delivery mostly in the U.S. and Canada — into a whole bunch of new markets in Europe and beyond.

For DoorDash, buying Deliveroo is all about growth.

“All these firms, their valuations are built on growth — growth and margin,” said Gad Allon, a professor of operations, information and decisions at the Wharton School at Penn.

Because app-based delivery is a low-margin business, companies have to keep growing.

“And after a while, there is a limit to how much you can actually grow in the U.S.,” Allon said. “People can only order that many times breakfast, lunch and dinner.”

DoorDash started out doing restaurant delivery, but you can also use it to get things from grocery stores and pharmacies now. The company makes 54% of all third-party app deliveries in the U.S., according to David Portalatin, senior vice president and food industry advisor at Circana.

“They're already the dominant player there, and they're already growing within that space,” he said. “So how would you accelerate that?”

One way: acquire a competitor and expand into other countries.

It’s a future-driven move, noted Cornell’s Alex Susskind.

“Third-party delivery is particularly important to the younger generation,” he said. “It's the right move to be making, to have a global footprint.”