Title: Why should we use master page when we have Iframes?
Tags: asp.net;iframe;master-pages
Question: We know master pages are a strong feature, so why are many sites developed using iframes? For every request an iframe is downloaded from the server which in turn increases the overhead on the server instead if we use master page only the ContantPlaceholder part is processed by the server.
Comment: By the way, based on your actual question, the title should be more like "Why should we use iframes when we have master pages?"

Here is the accepted answer: You are right, iframes are put extra load on the server and master pages are more efficient. But there are some cases where iframes are necessary. One of the biggest uses for iframes is to display content that is hosted on separate domain. For example, facebook apps are often displayed in iframes so that their content can be hosted on a domain.
Some legacy applications use frames extensively as a design model and include JavaScript to allow frames to communicate. This is generally considered bad design and can often be replaced with master pages in a way that is much easier to understand.
Comment for this answer: -1, master pages put extra load on the server, and iframes are more efficient.
Comment for this answer: But this neglects to consider that you have multiple requests only on the first load of the page. Every button you click subsequently requests only the iframe content ==> Much less traffic volume.
Comment for this answer: @Quandary iframes are entirely new (extra) requests. Each new request has to go through the entire page cycle. A master page is just one more step in the page cycle. So yes, a master page adds load to the server, but multiple HTTP requests is much worse.

Here is another answer: IFrames and the page that represents them are completely disconnected from one another.  The only way to have an IFrame and it's parent communicate is through client-side scripting.
MasterPages gives you the ability to set ContentPlaceHolders and use server-side code to manipulate the final HTML that is outputted.
Comment for this answer: Yes. It is widely accepted that IFrames should be a last resort for getting content to show up on your site. If at all possible, you should pull that content directly onto your page (via database, web service, etc). Using IFrames can also affect your search engine rankings, because it may not get indexed with site that the IFrame lives on.

Here is another answer: When you say "why are many sites developed using iframes," which sites are you talking about? I don't think I've seen an iframe-based layout since the nineties. Nowadays, iframes are mostly used to let third parties control advertising, to track history on an AJAX-enabled page, or as a workaround to the same-domain AJAX rule ("JSONP").
As far as layout goes, everybody uses some equivalent of Master Pages.

Here is another answer: Masterpages are way more SEO-friendly than any other iframe solution

Here is another answer: Simply put, a Master Page can do much more than just an IFrame.... IFrames can act as content holders but they cannot provide a template for how all your pages should look.
Master pages provide this template functionality along with other goodies.
In addition, IFrames are more expensive. Here is a link that discusses the topic.
http://my.safaribooksonline.com/book/web-design-and-development/9780596803773/using-iframes-sparingly/summarizing_the_cost_of_iframes
Comment for this answer: Why not ? They can add .css and .js files to the child page via JavaScript. Just as efficient as a master page in this regard.

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