On "Can I use" you can search anything that you use in many different coding languages, and check to see what browsers support it. There is also other resources such as showing the latest features so you can learn about what has been updated recently. Then they also provide a resource if you need to test your sites they provide a link where you can sign up and use to test your websites.
One way that you may need to use the data respresented is if you a designing a website and the customer uses a specific browser, then you want to make sure everything you use works properly on the browser.Then another way that the information will be used is when developing a website and you want to ensure everything you are creating works on all browsers. If you are ever not very familar with a property and how well it works with all browsers then this would be a helpful site to check out first. Finally, if you want to see the newest features then you can go to this website and check out the section just for that.
Transform
Flaot
When using "transform" in CSS, it is supported almost all browsers besides Opera Mini where it is not supported at all. Then if you want to use "float" in CSS, it is also supported in all browser, though there are a handfull that the support is unknown.
This website is useful resource when you need to check if want you are creating works with a specific or all browsers. It gives you all the different versions of browsers so incase you or somone else is using a older browser then you can see if is still functional on the browser.