Updated on August 5, 2017: I had to update this article due to drastic changes in appearance and navigation in Google+ among other things. Classic Google+ at the time of writing my original post (way back in May, 2015), retired in January, 2017. So the post is up-to-date and current now. Enjoy!
We have discussed the 2 reasons so far, why you should be on this amazing social media platform, in the previous articles:  An awesome and great interface and,  The Circles which provide powerful sharing options and flexibility. Lets go for the 3rd one now:
Communities are yet another powerful feature of Google+. Communities help you share with people of similar interests what you find interesting. For example, the communities on cooking may help you to engage with others on recipes. This video conveys the idea but the Google+ user interface has changed since then (more about it toward the end):
Video demonstrating power of Google+ Communities
Once you join a community, your Home stream is updated with its posts and you know what other community members have shared.
Communities bring you in contact with people who share your passion. You can exchange ideas and the latest news and explore new contacts. You can post photos, share videos, plan events and chat (or video chat) using hangouts with other members of the community.
Where to find Communities?
Assuming you are logged into Google+ on your desktop, you can find Communities in the menu on the left side:
|Desktop version of Google+ Communities section|
The top bar displays three tabs: Recommended, Member and Yours. The first one shows the communities recommended by Google+ from different areas of interests for you to choose. The second one 'Member' shows the communities you have joined. For example, it shows the communities I have joined - recently visited ones on top and list of all below that. And the last tab 'Yours' displays any communities you have started.
Phone or Tablet
Now it's even more easy to stay connected with your community on your mobile phone. Android devices come pre-loaded with Google+ and it can be installed on iOS devices.
If you are logged into Google+ App, you can see a tab strip at the bottom. It has following items: Home, Collections, Communities and Notifications. The picture below demonstrates just that:
|Google+ Communities on Phone App|
When you are in Communities, you can see the same tabs as on desktop toward the top.
There are communities you can join straightaway and start action and there are communities you need to request to join. Once your request is approved by the moderator, you are a community member.
One word of caution, Communities have rules and guidelines which they expect its members to adhere to, failing which the members can be blocked. One of the main reasons is spamming.
If I recommend you one community to start with, it will be: Google+ Help community. You can explore the posts and comments there which will help you in enriching your knowledge of the Google+ platform and the awesome features it offers.
So what are you waiting for? Look for a community around your passion or hobby, follow it, participate, collaborate, network with new people and have fun. Move on to the next and 4th reason: Google+ Collections. Enjoy !
Classic Google+, which was in operation when I wrote the original post, retired in January, 2017. Read this post from Google+ Product Manager announcing: "With this latest round of updates, we believe the new Google+ is really your Google+— designed around your suggestions, requests and needs. It also means it’s time to say goodbye to classic Google+ on the web, which we’ll be turning down on January 24." Since I updated this post to be in line with the new Google+ interface and features, it made sense to delete the below old screen-shot. What you see below is the Classic Google+ and if you have watched the video above, it shows it in action. Then, I thought of keeping it posted here for historical interest after all history gives us an idea how we and things evolved and transformed in all those years:
|For historical interest: Google+ in May, 2015|
Move on to the next (part of the series) and 4th reason: Google+ Collections.