Our 1965 house in Upstate NY had aluminum wiring. There is nothing inherently wrong with aluminum wiring. The issue is aluminum has a higher expansion coefficient than copper. The OUTLETS were designed for copper wire. The problem is circuits near the top end of their current capacity. The repeated cycling loosens the wire under the screw. Eventually, the loose wire can cause sparking.
We had an outlet go bad on the circuit for the coffee maker.
The pigtails are for for non-outlet connections (lights, switches, fans, etc.).
Outlets should be replaced with Al/Cu outlets. They are $2.99 at Home Depot.
Leviton 15 Amp Duplex CO/ALR Outlet - Ivory-R51-12650-00I - The Home Depot