Frankly there are quite alot of reasons why your forum or blog needs a guestbook.
But before that....
What Is a Guest Book?
A guestbook is also known as a guest book, visitor logs, or visitor's book. In times past and even presently it is a paper-based ledger or booklet with which a visitor  acknowledges a visit to a place,events, and leave details such as name, contact information and comments concerning the occasion. They are normally made available at funerals,naming ceremonies,confirmations,exclusive parties,hotels etc and it can be kept by the chief host for future reference or discarded.
Web based guest books also proffer the same as paper based ones except they are placed at certain pages plus can be kept for a longer period of time.

Now why you need One
Customer Support
Frankly with a guest book visitors or users and their needs can be met more easily and with a guest book only important messages need to be sent to admin others can  be placed on the site and if you happen to reply the comments you gain the confidence of your users.

Increased Traffic
Yup you read right by adding a guest book you increase the possibility of more views  by 20-25% cool right which means about 2 out of 10 users who visit daily will come back.

Builds a small Community
With a guest book a small community can exist where visitors can have their say and you are the judge to approve or disapprove.

Betterment Of Coding Skills
A guest book also helps you better your skills since you'll want to satisfy all your visitors requests thereby increasing your skills.

I think I'll hold the fifth for now
Happy Coding sign Wcoders Guest Book Web Coders Guest Book