If you are just starting to learn a language then it is useful if the teacher speaks your language and the language you are learning. So, for example. if you are a Chinese person and are starting to learn English then it is great if your teacher is also Chinese and you can communicate with them. I'm starting to learn Spanish and it would be great if the teacher spoke both English and Castilian Spanish.
At the higher levels ( B1 -C2), with adults, I find they want to practice with native speakers. They have already learned grammar until it is coming out of their ears etc. They can speak with other non-native speakers at work etc but they don't have the confidence and skills to speak with native speakers.
I often read about the debate between non-natives and natives. Many of the English speaking jobs state native speakers only and usually specify a country or countries the applicant must come from. Non-native teachers of English cry discrimination and state they should be allowed to teach and why shouldn't the student have a non-native teacher. These teachers forget that there are already heaps of non-native speaking teachers within those countries. So in China, you get heaps of Chinese teachers teaching English, in Europe you get heaps of Europeans from any country teaching English, in Russia, you get heaps of Russians teaching English, etc. So the students do have non-native teachers and can choose a non-native speaker if they wish. Many countries under work visas regulations can not employ a person from another country if a person in the country can do the job, so many non-native teachers simply don't qualify for a work visa.
Now to online teaching. Again there is the outcry of discrimination and the statement that the student should be allowed to choose. Well, of course, they can. Within their own country, they can go to non-native teachers, many of whom will teach online as well as face to face. Many choose to go online simply because they can get to speak with a native speaker. For parents, they see it as an added benefit for their child to be exposed to native accents and western culture. It also looks good on the kids resumes for entrance into the 'right schools'. Remember the online kids English classes are just an add-on to their normal school classes. The kids, of course, get exposed to non-native English speakers at school in their normal English classes. Adults who choose their teacher again often choose the native teachers because the want to understand the native accents and the relaxed way of speaking. They have usually learned grammar and a lot of vocab at school so need the speaking and listening practice. I have many who need English for work as they work for multi-national companies and have colleagues and clients who are native speakers. They feel that their English is inferior and it makes them look bad in the workplace so want to get used to speaking with a native.
Just one further note. Many may not be aware but the Chinese online schools and other Asian online schools use tons of Filipino teachers. Officially they are classed as non-native but many speak English as their mother tongue. The Chinese use them specifically because they are cheap. Usually, they pay them less than $4 an hour and I have seen ads offering them $1.5 an hr. Some online schools now mainly have Filipino teachers while others are slowly shedding their native teachers in favor of the ultra-cheap teacher.
Overall there are probably more non-native English speakers teaching English around the world than native English speaking teachers.