HE is a major "IP transit" provider, if you're a small to medium sized ISP, they are one of your choices in the market for buying absolutely necessary transit to reach other parts of the Internet. Grossly simplified, you as a tiny ISP get your own AS, and at least a /24 of RIPE, APNIC or ARIN IP space, set up your own router, and announce your AS and IP space to HE over a BGP session with them. Usually via a singlemode fiber crossconnect from within the same carrier hotel or datacenter.

Or if you're a colo/dedicated server company you can use them as an upstream. They also sell layer 2 transport services (built over MPLS) between any two of their POPs, for ISPs that want to establish a new POP somewhere and get 10 or 100Gbps connectivity between them.