Ian McCarthy, principal product manager at LinkedIn, wrote via the LinkedIn Blog that the feature came about after Group Moderators shared that they were looking for "easy, time-sensitive ways to generate conversations within the group." Apparently, such conversations weren't happening quickly enough after Moderators posted articles or blogged their thoughts. Polls was devised to get more conversation flowing.
Although McCarthy addresses Group Moderators in his post, anyone in a Group can create a poll. However, Moderators can restrict the creation of new polls from their Group Settings.
LinkedIn is trying out Polls out in Harvard Business Review's Group of 150 and as of press time, received 64 responses. That number is a strong participant-to-non-participant ratio, but the results were no doubt influenced in part by the novelty of the experiment. However, adding the Poll function addresses what's perceived to be a major issue for LinkedIn: The lack of activities there beyond networking and job seeking.
The ability to poll on LinkedIn is not new. However, until now, such polls have been public rather than aimed at Groups. The enhancement is the latest for Groups, which has evolved from merely a badge with no real functionality to small communities within the site.