Electrolytic caps dry up eventually. When there's enough load (=current) on them at the end of their life the last bit of electrolyte gets rather hot and is vented through the top or - more often - through the bottom rubber plug. With very little load they usually just dry up and cease life peacefully.
However, if these dried caps are not replaced they may cause collateral damage by not filtering out voltage spikes or not stopping other components from oscillating.
