It's a throwback to the older designs of the Atra and Trac II. Unlike the Sensor and later models, which use thin blade strips, the Atra and Trac II use full blades. Only the edges of the blades are exposed; the remainder of the blade body is housed in the plastic "above" the blade edge. This is why there is such a big area of plastic above the blades in those designs. This plastic drags over the skin after the blade edge has already removed the cream/soap/foam, which increases the potential for drag. The lube strip addresses this problem by lubricating in that area.
Once Gillette introduced the Sensor, with its more minimalist design, the lube strip didn't really serve a necessary purpose, since there is only minimal area above the blade. But because the strip had been a big marketing feature for the Atra, it makes sense Gillette would keep it, lest consumers wonder why the company was taking a step "backwards" with the new model. Leave it to Gillette to come up with a way to capitalize on it by having it change colors to tell the user when it's time to change cartridges.
