Most people who are part of the workforce, go through the recruitment process. Several things are done prior to landing a job, which would include composing cover letters, building one's resume, searching for companies that would best fit one's needs, and doing research on the job qualifications. Nowadays, it's both harder and easier to find a job because of social media. Jobs are posted everywhere, and the types of jobs are multiplying every second.
When you are submitting your resume and cover letter, most feel compelled to try to sell themselves or be noticed by being funny, or even putting something unique on these documents. However, we always have the hesitation of doing so, since these are official document that may or may not reach high level management.
So where do we get a chance to sell ourselves, aside from the actual interview? Even if we have time to prepare for it, we will never know if we will be able to say everything we need to say.
LinkedIn is like corporate Facebook + Twitter. Nowadays, it is one of the top recruitment sources, and although when you formally submit your resume to recruiters of your target employers, you have full control of your profile, while still looking professional.
I never used to have anything in my summary tab on LinkedIn, until I realized it is probably the most powerful part of your LinkedIn profile. Without your summary, all you have is your resume, without that "personal touch" that you want employers to see prior to the interview.
If you want to see examples of LinkedIn summaries, feel free to check out Andy Foote's article about 3 Stunningly Good LinkedIn Profile SUMMARIES.