For those of us who started early with digital cameras, the original 5D was a watershed camera — a full frame camera that we could (perhaps barely) afford. A 5D was my first full frame body. It wasn’t perfect (it was a dust magnet, it wasn’t particularly fast, and it lacked capabilities we now take for granted), but it was a significant step up from teh 8MP APS-C bodies that were the contemporary alternative.
The 5D began to show the potential of the 5D line, with much higher resolution, live view (a boon to landscape photographers), video, and other improvements. For those like me, who used these cameras for landscape photography and similar subjects, the 5Ds and 5DsR were another big step forward, offering resolution that Canon can’t match with its current mirrorless bodies. (I still rely on my 5DsR.) The 5DIII and 5DIV continued to advance and distill the 5D concept.