> You don't need analytics to know that you should build a responsive website
You do need to know if you have users using their fridge's custom browser developed by some intern long since retired in 2012 that doesn't support HTML5 properly and needs weird hacks to actually be responsive.
> resource usage small enough for old devices
Which can prevent you from adding new features, or even just video in good quality which can choke an old device's browser. Graceful degradation is nice, and should be preferred whenever possible, but sometimes not possible. So knowing how many, and if any, of your users are using such things you have to go out of your way to support, is important.
You do need to know if you have users using their fridge's custom browser developed by some intern long since retired in 2012 that doesn't support HTML5 properly and needs weird hacks to actually be responsive.
> resource usage small enough for old devices
Which can prevent you from adding new features, or even just video in good quality which can choke an old device's browser. Graceful degradation is nice, and should be preferred whenever possible, but sometimes not possible. So knowing how many, and if any, of your users are using such things you have to go out of your way to support, is important.