what's the difference between iCloud.com and iCloud on my devices?

iCloud.com is a domain designed by apple engineers , and iCloud on iPhone is set up Set up iCloud on your iPhone, iPad, or iPod touch - Apple Support by signing in with Apple ID and password with its account credentials Sign in with your Apple ID - Apple Support

Read What is iCloud? - Apple Support

Both, icloud.com and iCloud on your devices are providing access to the data you are storing in iCloud.

• To access the data and documents with at www.icloud.com, you can use a web browser on any device. The web browser will allow you to launch apps to work with www.icloud.com, in a limited way. You need internet access to use iCloud an www.icloud.com .
• On your devices you will have dedicated apps, optimised for this device type, to manage and access your iCloud documents. This will usually be more versatile and faster, as the devices can mirror iCloud and store shadow copies of your cloud documents. This will save a lot of network traffic, and thanks to the shadow copies, you can work even offline.

I am using www.icloud.com only, when I am not having any of my devices with me and need to use a borrowed computer.