They offer a lot more than just VPS services. But compared to the likes of AWS, Azure and GCP their pricing structure is simple and predictable. That coupled with the straightforward management interface, team management & access control systems make it a very inexpensive and approachable solution. It just has a really low barrier of entry, and its partially why I chose them for some projects with my current employer. There isn't enough rope in the management UI to hang yourself with, and I can handover the lot to my replacement even if they're not particularly clued up on system administration.

So to answer your question, I see DO as offering a subset of what AWS/Azure/GCP do, with the cost and complexity savings to reflect that. So those larger cloud providers are what I consider the higher end.