Megaport's main business is automating the bring up of...

Megaport's main business is automating the bring up of connections between two networks.

At the start the main users were generally ISPs who needed to create connections between each other, e.g ISP X doesn't have a presence in a certain city so they contract with ISP Y to connect that user and present the users connection over MegaPort back to ISP X.

Before Megaport the two ISPs would need to figure out which datacenters they both had equipment in and arrange someone to connect a cable between their two racks. Using Megaport they can avoid having to build physical interconnects between each other. (Other methods such as internet exchanges also existed but Megaport automated and generalized it)

As cloud computing has taken off the same concept has applied to AWS, Azure etc.

While it's possible for such traffic to be purely exchanged over the open internet, it's not always cost effective or as flexible.

AWS charges significant $$ for outbound traffic (from AWS to the public internet) which can be avoided by directly connecting to them, either using Megaport or through other means (e.g if you are a really big company you might skip Megaport and interconnect with AWS directly/physically)

If your company uses an application in AWS daily it becomes attractive to use Megaport so you or your application provider doesn't cop the AWS traffic charges. It can also provide more flexibility to bridge "on-premise" networks with cloud resources.
Such a company probably already has their own private network interconnecting their offices so can use Megaport at their main datacenter to hand off traffic to AWS.