Originally Posted by aidantrent

IFrames are most frequently used to load code from different sites, like ads and such. Sometimes the loaded code is malicious, and there's no way to check since the source of the iframe is generally dynamically generated content. Most TGP owners won't allow them in galleries they link to, and my own adult search engine ignores any galleries containing them.

As cartelcash mentioned, there are better methods of doing frame-based layouts; the main reason people still use them is include something malicious in an otherwise TGP-friendly gallery. Just avoid them