There are many reasons an outlet may need to be replaced.
In older homes, basic wear from years of use may be a reason. The more an outlet is used, the more likely it is that the receptacles will get worn or damaged. One sign of this is if plugs fit too loosely in them or if there are cracks receptacles or the outer plastic casing.
Although these may seem like small issues, problems like exposed wires can put you and your family in danger of electrical shock. Loose plugs are more than just inconvenient; they can lead to overheating and become a fire hazard.
In older homes, outlets also lack safety features such as grounding (the bottom hole found in some outlets) or GFCI (usually found in bathrooms with test/reset buttons).
The best way to keep your family and home safe is to contact a qualified electrician to inspect your outlets and replace them if necessary.