What the benefits of subdomains in SEO ? Are they really considered as separate sites by google and other search engines ?
Well first of all, there is a money saving as opposed to buying individual domains, and that is a good thing. As far as SEO, I have found that subdomains can rank just as easy as an EMD (exact match) in Google. I have many and no complaints
You save money for one. Another benefit is that any penalties on one subdomain don't affect another. Subdomains are good for certain ways of organization your site. You generally don't get SEO benefits from the root domain though.
there is no benefit in SEO. subdomains are considered as seperate site so what you put under a subdomain is not part of your main site content. people use subdomains for various reasons. main reason is to have separate sites for separate categories. think of blogger, wordpress for example. they give you subdomains where you can build your own site which is a totally new site
What if we use sub domains with unique content and linking to our main domain, can those links be considered as fruitful links?
No it cannot be. It is the reason why different web properties are used to make blogposts and build linkwheels.
Most businesses don’t like to use subdomains because they feel like they give them a disadvantage in many areas, such as having a prefix name before your web-site, being ignored by bots and index spiders in search engines and all in all, it just doesn’t look right. So let’s check our facts on subdomains.
One advantage is that the subdomains can distribute traffic to different computers without any problem. www.example.com could be indicative of a web server virtual hosting, where blog.example.com ftp.example.com or can point to a computer or a LAN to the Internet such as DSL.
There is nothing big benefit in SEO. I also agree with sultanofseo that sub-domains are considered as separate site and that contents are not part of your main site content. Some people use sub-domain for saving money and for separate categories.