What will be the difference in using a free WordPress theme and a premium theme?

I'm looking to develop a website like this, it's the company website for which I'm presently working. So, I would like to know whether it is good or not if I choose a free theme to start developing. What are the features that I will get if I buy a premium wordpress theme? I think I can develop with css even with a free theme. What's your opinion? I'm a beginner and an intermediate in developing. I have been in the academic writing industry since 7 years. Please let me know.

Thanks a lot.