Twitter is can be valuable for marketers in fields where constant updates are welcome. Companies like Apple, Google, and IBM can use Twitter to quickly disseminate news updates and links to articles that introduce or promote their product/service. Twitter users will actively search for these company accounts and will follow religiously.
For those marketers from companies that are not so much in the public eye, Twitter can be useful for learning from thought leaders in their industry, promoting articles and blogs, and gaining followers that could potentially act as business partners.
Many articles and Webinars on HubSpot explain using Twitter for Marketing and PR in an easily understandable way -
http://www.hubspot.com/twitter-for-marketing/. Twitter.Grader.com is a very useful site for understanding your Twitter grade and how to improve your profile.