When you upgrade, the necessary and relevant files are replaced. The old files are deleted, while the new file takes it's place. Some files may not even change.
For a clean install, you want to delete ALL the existing partitions, unless you have a system restoration partition that you wish to keep in case you want to clean install back to your previous windows version, in which case, delete all the OTHER partitions, but leave that one alone. You will have the option to do this when you get to the point in the installation where it asks where you want to install windows. I recommend deleting all partitions and installing to the unpartitioned, unallocated space. Windows will create the necessary partitions and perform any necessary formatting of partitions.
Alternatively, not to send anybody away from OUR website, but if the above tutorials don't work for you, specifically AFTER you upgrade, have installation media ready and are ready to do the clean install, you can use this guide as well:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
Deleting the existing partitions deletes ALL the information that was on the drive. It will essentially be blank. The biggest reason to do this is that it allows you to:
start fresh
remove previous boot partitons that may interfere with the smooth operation of the existing OS installation
recover drive space
make certain there is only one boot partition and that it's of the type required by the version of windows you're currently installing.
Probably some other reasons too, but I'm getting too tired to think of them right now.