We all use them, everyday, usually multiple times a day. Whether its using an item that remains plugged in all the time, like a television or microwave, or plugging in a cell phone charger, tablet, or laptop computer at some point in the day it will make our lives a little easier. We sometimes take it for granted, but just cant live without the uninspiring receptacle. It has changed (a little) since it was first patented in 1904 with its two wire design but its use and need for replacement haven't really changed at all.
A receptacle needs to be replaced when it becomes worn and the connections become loose. When a cord is plugged in, it should fit tightly, with the base of the cord holding firmly against the face of the receptacle. If the cord is sagging, or the weight of the cord begins to pull the prongs out of the receptacle, it is time to replace it. If it is not replaced the loose connection could arc, generating heat that could cause the receptacle to melt especially with a high load item like a space heater. An electrical arc (which burns hotter than the sun) can also cause a fire with just one spark igniting a nearby combustible material.
Some receptacles should be replaced. For instance, if you are plugging in an appliance with a grounded cord end into a two wire receptacle with an adapter (like a refrigerator), you should consider replacing the receptacle. The refrigerator was designed to have an equipment ground. Although it will work without the ground, the adapter creates another fail point, adds weight to the cord end and moves it away from the connection, which may cause it to sag like the issue above. The ground is commonly used to bond the metal casing of the refrigerator (to prevent shock) and to protect the refrigerator from surges that may damage its components. You should seek the advice of a licensed electrician before replacing a two wire receptacle, because a two wire receptacle cannot just be replaced by a three wire type. If there is a suitable ground path present, the electrician can bond the receptacle to the box to complete a grounding path. NEVER bond the ground screw to the neutral to fake a grounding path. There are other acceptable methods listed in the National Electrical Code but providing a ground path is the best option for protecting the equipment and the people using it. Unless you need to plug in a three wire cord or your receptacle is damaged or worn, there is no reason to replace a two wire receptacle. Most of what we plug in today is manufactured with a two wire configuration and the manufacturers have designed it to function safely without the equipment grounding path.
Another type of receptacle that should be replaced is any receptacle in a wet area (such as the kitchen counter or bath) on the exterior, in a garage, or unfinished basement that would require GFI protection, that is not currently protected. Ground Fault Circuit Interrupters save lives.
If you have children you could also consider replacing your existing receptacle with tamper resistant receptacles. This will eliminate the need for the protective inserts that can wear out the terminals inside the receptacle causing cords to sag. Tamper resistant type receptacles have shutters permanently installed to prevent a child from pushing anything into the device. Both shutters must open simultaneously for them to open.