It took me a long time to get Tumblr, like many others I wondered what it was for, why would I use it instead of a blog? All a bit of a struggle for a computer geek to understand, we have so many tools on the internet such as wikis, blogs and forums that are designed to solve problems and bring us closer to realisable outcomes. We developers share traits with scientists and engineers in that we look at the world in a logical way looking for cold rational answers to questions. The delight of Tumblr though is not really strongly related to this aspect of life. I only understood it when I stopped worrying about its purpose and started to enjoy it, using it as an inspirational media scrapbook.
Yet we all have those moments when we just like the look of something, how it is presented or the experience of interacting it. These moments can't be described as entirely rational, and there is nothing wrong with that, it is just that those moments have appealed to a different part of the brain I suppose, a more artistic side. My Tumblr account is a scrapbook of things I like or find interesting, often for no particular reason. You can use lots of different ways to get content into your Tumblr account, one is the very handy Tumblr Bookmarklet, this just sits at the top of your browser and when you see a picture, video, quotation etc that you like you can just use this tool to record it. Pages like the Tumblr Radar show popular content from other Tumblr users which can be interesting (though be careful as some content may be targeted at adults), when exploring other users' Tumblr blogs you will see a link that lets you "reblog" an item giving you an easy way to copy that content to your site, with a credit to who found it. Sometimes I put pictures up that I have taken on my mobile phone, like many people I have a phone with a camera and can take pictures of anything I find interesting or unexpected while out and about, it is also possible to send this straight to Tumblr from your phone.
You can can follow other people's Tumblr postings to see what they have been up to, you can't leave comments though, which is maybe a good thing in this context as maybe it is a little bit like an art gallery where people can view the pictures and interpret them in their own way, particularly with something that mght be quite personal like a collection of things you find are inspiring. This not to devalue debate or feedback, but when a thought is half formed a hasty judgement might undermine a useful idea.
It is my virtual scrapbook, a place to put things that might inspire me to be more creative. As such its benefits are harder to measure than a lot of tools, you could never apply any metric to measure its value for different people. So why does any of this matter to a developer? In a way getting in touch with our creative sides is becoming more essential. While it is true that in the web world at least web developers and web designers are two different professions, one does the science, one does the art, such a stark division might become unsustainable as expectations raise about what a site or program is like to use. People are not content with just a tool anymore, they want an experience, a whole package that does what they need and leaves them feeling good about using it. I'm no fan of the iPhone or Apple products, but it is undoubtable that this is something Apple have demonstrated can be a powerful concept. Using sites like Tumblr we can stimulate our creative side leading to the idea of a good experience using a site or product being much more deeply ingrained. For now it is my irrational dumping ground of "stuff I like".
Re: The joy of Tumblr
I still don't grok tumblr, but I have been using posterous on and off, and it serves a similar role. It has some fabulous easy features for posting direct from email and will then reblog/post onto other blogs and twitter. Very useful for uploading photos and just commenting on other's blog posts. I use it mostly from the iTouch or from Google reader to save those horrible copy and paste problems!
I agree about your thoughts on being creative too, we all need a little visual inspiration now and then!
Re: The joy of Tumblr
yes, took me a while to get it too. Scrapbook is exactly right - it requires zero physical effort using the tumblr toolbar button, but more importantly zero cognitive effort. The way my blog has developed I need to write something insightful (ok, maybe not always) about a subject. Tumblr is just for dumping stuff into a big bucket as you browse around. I'm not sure it's really for an audience as much as my blog is, more harvesting for my own purposes, but while I'm doing that I may as well share it.