So you have backed up your footage and you are now ready to edit. If you have the computer to handle it and you know you will be the sole editor from start to finish on the project, then, great, proxies are probably not necessary. But, if your computer struggles at the sight of compute-intensive H.264 files, especially 4K or higher ones, or if you have even the inkling that the edit might be sent to someone else for coloring, audio, final touches, whatever, then proxies are a very good idea.
While you can double-click on most content to open it in the source monitor, you can only open most content. Sequences, for example, will open in the timeline.