SAP runs on Windows, Linux and several flavours of Unix. Which a company uses depends on many factors. Cost and existing skillsets are the two main factors.
Should you finish the Red Hat cert? I would say that's up to you. Some companies have a large enough Basis team that they can split into OS, DB and SAP specialties. Some are smaller and have people doing all of it. The cert might help you find a job, but if you are looking to get into "proper" Basis it won't help a lot with that, in my opinion.
Steve.