It's helpful in web development, it allows you to target specific browser versions. For example if I use a new feature on my website which is only supported by recent browser versions, I can inform the user about the incompatibility and prevent complains.

I also don't think Google needs that info, there's a million things to criticize them for but this is a bit silly.

Feature detection is more reliable than version checking, as it can more correctly support a wider range of browsers (that you didn't think to version-test for). There might be some cases where that's tricky, but it's generally straightforward. Tools like https://modernizr.com/ can help with that.

You don't think Google do browser fingerprinting? As a sibling comment suggests: feature detection seems superior.

If a server know the client's browser version, it can serve JS polyfills to older clients and smaller files without the polyfills to more recent clients. Here is a polyfill service hosted and maintained by the Financial Times: https://polyfill.io/v3/

Apart from IE5.5 I've only used polyfills against major browser number, do current polyfills really look at minor browser number?

It's the biggest and most obvious thing to fix (especially with JS disabled), so clearly it's silly to worry about.

Chrome with version is enough not the rest of the stuff.