I've always wondered whether network interfaces that have these flashing lights flash as a gimmick or do they actually indicate the flow of traffic? Perhaps one flash per packet in or out? I wish I could remember what my call up modem looked like to make a historical comparison too.
TL-SG105E
It's an activity indicator, it doesn't necessarily correlate to anything other than transmission or receiving is occurring. I would also not assume that they work the same between brands. Some devices may have a separate link light, others may not. Some will have a combined tx/rx light, others separate.
It's not a gimmick though, they're still useful for that purpose.