Frames are no longer part of the HTML5 standard because they cause accessibility issues, and there are ways to create similar styles with CSS and the
IFRAME element that are more accessible. But I still see people using frames periodically. And it always surprises me. One reason you might consider changing away from a framed site is because of SEO. It is possible to optimize a framed site for search engines, but it's harder and many people forget to.
Read the full article: Frames and Search Engines