1.Easier to develop with — as mentioned above, you get some functionality for free, plus it is arguably easier to understand.
2.Better on mobile — semantic HTML is arguably lighter in file size than non-semantic spaghetti code, and easier to make responsive.
3.Good for SEO — search engines give more importance to keywords inside headings, links, etc. than keywords included in non-semantic <div>s, etc., so your documents will be more findable by customers.