I would like to know how signature in email is useful.
Well, it provides your contact details etc. But if you have images in your sig and you send attachments too, your recipients end up downloading all your sucky sig images along with your attachments, which is a major PITA. So stick with a text-only signature.
If you can put your website’s link in your E-mail signature, this will be helpful for SEO.
Signature in email is useful when you create a signature in email you do not need to type the same signature again and again when you compose the mail box to send mail repeatedly.
Its no needed but its helpful for Search Engine Optimization!.
No, it’s not. We just had that discussion 
See http://www.sitepoint.com/forums/showthread.php?775009-Email-Signature
How this will be helpful to your SEO campaign? e-mails are password protected and Google or any other search engine cannot crawl password protected stuff, So how would e-mail signature contribute in your SEO campaign?
Email signature would be a good option for brand promotion. When the recipient receive useful emails from you he/she will check the name of the sender (specially in case of email marketing). It help your recipient to remember your name or your company name.
I do not think signature is very useful in email but yes some times it provides traffic.
Sometimes I wish that posting on the Internet wasn’t anonymous, so that people who write posts like “an email signature is good for SEO” can be named and shamed, and never work in SEO for the rest of their lives.
To be constructive, SEO is entirely different to site promotion. No search engine crawler is going to read your email, therefore it is of no use to search engines. It may, however, result in a few more people clicking through to your website.
The point of the web addresses in your signature is that the person receiving the email already knows (as if they don’t then it is spam) you and so is far more likely to take a look at those sites if you tell them the sites are yours than some random stranger would.