When the iPod came out you largely had two options for carrying your music collection on the go. You either carried a binder of CDs, or you had some niche player like Mini-Disc or an MP3 player. Both alternatives were expensive and had limitations similar to a CD in terms of number of tracks you could carry.
I had an MP3 player on either side of 2000 that was slightly smaller than a deck of playing cards that could use Smart Media flash memory cards. The largest card at the time was either 16 or 32mb and was enough to hold 1 album at near CD quality.
Creative's Muvo was a weird form factor that was larger than an iPod. It had a horrid interface both on device and for loading music. It's only grace was that it was slightly cheaper than an iPod and didn't need a Mac with FireWire. Although iircc this was pre USB 2.0 so not having FireWire would mean loading music took forever and a day.
The iPod allowed you to carry most, if not all, of your music collection in a package slightly larger than a deck of playing cards. And it had a fantastic interface for navigating music on the device.
This was at a time before most people had laptops and if you had a PC it was at home and used sparingly. The iPod was such a compelling mobile computing device that it drove adoption of the iMac. Apple would eventually release iTunes for Windows and USB support but that was many years later.