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I have to install Ubuntu on a PC and I saw I have to make a bootable LiveUSB from the Ubuntu iso file. However, I can't understand what's the technical difference between making a bootable LiveUSB with a program like Rufus, and copying a .iso file on a USB. Why is not enough to copy it?

If you want to install Ubuntu, you need to make a bootable LiveUSB.

iso is an image of a bootable disk.

If you simply copy an iso file to a USB disk, you won't be able to boot from it.

A bootable media has specific data in specific sectors, so BIOS or UEFI can start a system from it.

you have to burn it to the flash drive using something like etcher

Creating an UEFI bootable Linux USB stick

We will create a bootable USB stick with Linux by starting from a Linux distribution’s ISO. Since we want to create a USB stick that will be able to boot a UEFI system, we will require an ISO that can do this. The ISO requires a special EFI bootloader.

Make sure that you download the AMD64 version of the ISO! You can download this ISO from the following link:  http://releases.ubuntu.com/18.04/
To create the bootable USB stick from the ISO, we use the Rufus tool on Windows. You can download Rufus from official website https://rufus.akeo.ie and version is Rufus 3.3.400 whose size is 0.98 MB.

Use Rufus as follows:


1. Click “Select” to order to browse to the location of Ubuntu ISO file and select the same.

2. Set “Partition scheme” to “GPT” which is the default partition scheme for UEFI computer.

3. Select “Target system” as “UEFI (non CSM).

4. Set “New Valume Label” as “Ubuntu-18.04.2-desktop-amd64-Bionic Beaver”

5. Select “File system” to be “FAT32 (default)”

6. Set “Cluster size” to “4096 bytes (default)”

7. Enable “Qick format” and select “2 passes” as show in the screenshot below.

8. Make sure that Rufus hasn’t changed the partitioning scheme after you have selected the ISO.

9. Click Start button to prepare bootable Ubuntu USB stick.

Figure-1: Ubuntu 18.04.02 Bionic Beaver

When Rufus is ready, you will have your USB stick with a UEFI bootloader. Booting a machine with this stick allows you to boot Ubuntu 18.04.1 LTS live session so that you have access to all default Ubuntu utilities.
Ubuntu ISO files can be downloaded from the following URL: http://releases.ubuntu.com

How can I upgrade ubuntu using bootable USB?

You just need to access bios of your PC, and boot from bootable USB, then select "install Ubuntu"

Then a bunch of options will appear (see the image below). Select "upgrade ubuntu" . Done

Figure-2: Installation type

You don't burn an ISO to a UFD. You do that with CDs and DVDs. Basically you don't need Rufus or any 3rd party USB booting tool as so many keep insisting.

If you are making this from Windows 10, first of all format the UFD with FAT32 using diskmgmt.msc from a command prompt (cmd) with administrator privileges. You right click on the UFD and format it as described. Then mount the ISO file by right clicking it and choosing mount option. When open just copy the files over to the UFD and it will be bootable.

The .iso is a disk image file, and if it's directly flashed to the disk, your computer won't be able to read it as bootable. The flashing just reads the .iso and puts it in a format that the computer can boot from. In a way, it's like a .zip archive in that the stuff it contains has to be extracted to be usable.

You can simply burn the .iso file of OS into DVD or USB and with that you can run an OS and do your job , but why ???
Many reasons like : One of your important OS file is missing and your OS won't work , so you can boot for example Ubuntu from USB and go to that file and undo the changes or fix it and etc.
Sometimes you need to do something and you can't be able to access your laptop to do your job , by having a bootable USB that has Ubuntu on it you can simply run ubuntu and work with it without installing ubuntu on that system (the OS boot from your USB not Hard-drive)