Joining a service club instead of volunteering individually has both
pros and cons from the volunteer's point of view.  Here are a few
people's takes on this question.

--------

 * Service clubs, because they can bring a great deal of man-power to
   bear at a time, often tackle different kinds of service projects
   than individuals volunteering directly with an agency.  (For
   example, run a fund-raiser, or perform renovations in a shelter
   which requires turning out all the residents for a day.)  There may
   be a stronger focus on specific EVENTS rather than helping with
   day-to-day operations.

 * Service clubs often have established areas or charities of
   emphasis; this gives focus to their service program, but it may be
   difficult for someone to find an organization whose service program
   matches his or her interests.

[Source: Ping Huang <pshuang@mit.edu>.]

--------

Great question, Ping!  It gets asked at my Rotaract club from
time to time, at least the general idea is discussed.  Basically,
we think that:

* the club gives you a chance to experiment with lots of different
  service activities to find something you really enjoy
* you may find that you simply enjoy the variety and chance to help
  out lots of different pet causes instead of being devoted to just one
* if you don't have a lot of time, you have a lower expectation of
  committment -- less guilt if you don't show up for the dog wash,
  lots of club members did
* the club is constantly changing, new people, new projects
* you can bring in your OWN interests and get others involved, an
  entirely different experience from being drawn into a group yourself
* for some, the networking is important -- you're connected to lots of
  different organizations, or meeting people in lots of businesses
  compared with solitary voluntarism
* the resources of the organization can be a factor
* for Rotary/Rotaract, some of the attraction seems to be
  participating in *worldwide* activities like PolioPlus (RIP Jonas
  Salk, he died with polio's complete eradication in sight).

[Source: Daniel A. Hartung <dhartung@mcs.com>.]