If you do not have your own domain, that means somebody else has control over your site and not you.

If you use site40, for example, they can delete your site at any time.

If you are using your own domain, you retain the authority you have built up for the site.

Buying a domain will help you in great amount. A good domain means good seo which means good indexing in Google which means more traffic to your site. If you buy domains like .com or .net then your site may also become eligible for displaying like adsense.
A domain will help in highlighting your blog.
I would recommend you to buy a domain and it will be more better if you buy a .com one.

PLUS.. buying a domain will make the url shorter and you will feel proud to tell others like your friends that you got your own website.
For example,,
I also run a blog now just see the difference in both the cases.
http://horribletech.blogspot.com
http://www.horribletech.com

You just see the difference..
in the first one there will be no WWW because its a sub-domain but the second has because its a top level domain name.

ABSOLUTELY, you need your own domain. Your blog is your BRAND. Social media and great content will get you to page one.

Yes, I think all website owners should have a proper domain as it gives off a sense of professionalism to the reader/user. Normally, proper domains rank better on Google, but that's just from what I've seen.