in plastics have specific purposes - usually to render the plastic soft and flexible, when it would otherwise be hard and more brittle, in other words, a material of lower molecular weight added to a polymer to separate the molecular chains.
There are hundreds of plasticizers (check out Merck's website). The plasticizers
in liquid form are generally somewhat like turpentine or paint thinner in consistency. The can cause plastic to swell and become flexible in this way.
Plasticizers (because they are a liquid) can also evaporate. This is what happens to cheap vinyl over the years (gets brittle and cracks). Your car upholstery needs to survive a 250°F environment and not lose flexibility, so they use more expensive plasticizers that won't evaporate so easily.
Armorall is a cheap plasticizer that, while softening your car's vinyl, will also evaporate all over the inside of your windshield when it gets hot. Armorall also has poor soapy water resistance and will wash out.
There is a difference between a plasticizer
and a 'slip agent' added to the plastic. Slip agents are added to plastics to help forming during extrusion (such as blown films to make polyethylene baggies). Without the slip agents, the plastic would rupture before being blown into a bubble.
Plasticizers for concrete are added to improve workability without compromising the strength very much.