Flexible. When you have the time, you go online. For example, the content is usually posted for a certain time period. You get to go in, morning, noon or night to read through the context and then complete the assignment. Downside - you have to have good time management skills. Otherwise you will fall behind.
One-on-one instruction. The teacher who is facilitating the course is available to answer your questions. Now the way I have seen it is that if you have a question or trouble understanding something, you need to let the teacher know. She will schedule an appointment time good for both of you. She will send you an email with a computer link that will connect your computer to hers. She will use software like Elluminate that has a "whiteboard" and answer your questions until you understand. These meeting as often individual or small groups. You connect via computer and phone. Downside - you are the one who has to reach out to the teacher to ask for help. she won't be able to read your body language to see what you don't understand.
Self paced. Most courses are built in modules. Once you finish one, you can move on to the next. No need to wait for the teacher or the other students any more. Once your finished, your finished. Downside - some people like to work with others. They miss the socializing with their friends.
Cost. Some states offer these middle and high school classes for free - instead of attending regular school. Check with your local school's guidance counselor or district offices. Downside - some times there is a fee or if you don't complete you have to pay the full price. Be sure to read and understand the fine print.
Frees up your schedule. This is good if you have to work. Since you can do you classwork when you want, you will be able to work more or different hours than those who go to traditional schools. Downside - time management. Don't over do it so that you are so tired when you get home you don't do your school work.
Studying online is very beneficial nowadays because it is more convenient and It allows students to work and learn at their own pace without the unyielding time restrictions of traditional schools. Students are more flexible and they find online education more accessible.
I think the main advantage of online schools is that you can do homework online. You have no need to spend all your time doing homework or writing essay. You are free to do whatever you want and get educational level. You can study when you want, you can make plans and your can be sure that you won't have any problems if you'll skip some lessons.