Improve the "What is Rerun?" documentation (rerun-io#5920)

### What

[Rendered](https://github.com/rerun-io/rerun/blob/emilk/what-is-rerun/docs/content/getting-started/what-is-rerun.md#what-is-rerun-for)

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5920?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5920?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5920)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

README.md

@@ -16,6 +16,10 @@
 
 Use the Rerun SDK (available for C++, Python and Rust) to log data like images, tensors, point clouds, and text. Logs are streamed to the Rerun Viewer for live visualization or to file for later use.
 
+* [Run the Rerun Viewer in your browser](https://www.rerun.io/viewer)
+* [Read about what Rerun is and who it is for](https://www.rerun.io/docs/getting-started/what-is-rerun)
+
+### A short taste
 ```py
 import rerun as rr  # pip install rerun-sdk
 

docs/content/getting-started/what-is-rerun.md

@@ -38,6 +38,32 @@ That's a big question for a welcome page. The short answer is that
 Rerun goes to extreme lengths to make handling and visualizing
 multimodal data streams easy and performant.
 
+## What is Rerun for?
+Rerun is built to help you understand complex processes that include rich multimodal data, including 2D, 3D, text, time series, tensors, etc.
+It is used in many industries, including robotics, simulation, computer vision, or anything that involves a lot of sensors.
+Let's look at a more concrete example:
+
+### Example use case
+Say you're building a vacuum cleaning robot and it keeps running into walls. Why is it doing that? You need some tool to debug it, but a normal debugger isn't gonna be helpful. Similarly, just logging text won't be very helpful either. The robot may log "Going through doorway" but that won't explain why it thinks the wall is a door.
+
+What you need is a visual and temporal debugger, that can log all the different representations of the world the robots holds in its little head, such as:
+
+* RGB camera feed
+* depth images
+* lidar scan
+* segmentation image (how the robot interprets what it sees)
+* its 3D map of the apartment
+* all the objects the robot has detected (or thinks it has detected), as 3D shapes in the 3D map
+* its confidence in its prediction
+* etc
+
+You also want to see how all these streams of data evolve over time so you can go back and pinpoint exactly what went wrong, when and why.
+
+Maybe it turns out that a glare from the sun hit one of the sensors in the wrong way, confusing the segmentation network leading to bad object detection. Or maybe it was a bug in the lidar scanning code. Or maybe the robot thought it were somewhere else in the apartment, because its odometry was broken. Or it could be one of a thousand other things. Rerun will help you find out!
+
+But seeing the world from the point of the view of the robot is not just for debugging - it will also give you ideas on how to improve the algorithms. It will also let you explain the brains of the robot to your colleagues, boss, and customers. And so on. Seeing is believing, and an image is worth a thousand words, and multimodal temporal logging is worth a thousand images :)
+
+Of course, Rerun is useful for much more than just robots. Any time you have any for of sensors, or 2D or 3D state evolving over time, Rerun would be a great tool.
 
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