You're right, both versions of straight blade plugs (NEMA 1-15 and NEMA 5-15 in North America, for example) don't offer different functionality in terms of carrying more power, data, or offering locking capabilities. However, there's a historical and compatibility-related reason for their continued existence, even if it seems redundant.Here's a breakdown: Legacy Compatibility

The ungrounded NEMA 1-15 (two-prong) plug was the original standard for low-power devices. Millions of devices and outlets using this standard already existed when the grounded NEMA 5-15 (three-prong) was introduced for safety. Completely phasing out NEMA 1-15 would have been incredibly costly and disruptive. Many older buildings and appliances weren't designed for grounded outlets. Manufacturers continued to produce NEMA 1-15 plugs for devices that were considered inherently safe or double-insulated, where grounding wasn't deemed necessary. This allows older appliances to still be used in newer homes and newer appliances that don't require grounding to be used in older homes (using an adapter).

Double Insulation

As you mentioned, modern double-insulated appliances are designed to be safe without a ground connection. These appliances have two independent layers of insulation to prevent contact with live conductors. This design makes them safe to use with ungrounded outlets (NEMA 1-15). Manufacturers can save on costs by using NEMA 1-15 plugs for double-insulated appliances, as they don't need the extra grounding wire and associated components.

Why not eliminate NEMA 1-15 altogether? Backward Compatibility

Simply banning NEMA 1-15 would create a situation where many older, perfectly functional, and safe appliances would become unusable without adapters. This would cause significant inconvenience and waste.

Cost and Disruption

Requiring all new appliances to have grounded plugs would increase manufacturing costs, even for double-insulated devices. Retrofitting older buildings with grounded outlets would be extremely expensive and disruptive.*

Perceived "Good Enough"

For low-power, double-insulated devices, the perceived safety benefit of mandating grounded plugs is not considered high enough to justify the cost and inconvenience of a complete transition.

In summary:

The coexistence of both plug types is a result of historical evolution, cost considerations, and the development of double-insulation technology. While it might seem redundant, it allows for backward compatibility, cost-effective manufacturing of certain appliances, and avoids large-scale retrofitting projects.Essentially, the NEMA 1-15 plug survives because it's "good enough" for certain applications, and completely eliminating it would cause more problems than it solves. Flag for review