spark plugs make explosions.
If you made an average of 800 explosions a minute you were awke, you'd wear out.
Same with plugs. As they age, they become worn. If the engine is in bad condition, then the spark plugs could be dirty, grimey, oily, melted, and a wide range of other things. Changing them can tell you a lot about the cars condition. Plugs also have a minimal imnpact on performance. Mine were changed today, and the car was noticeably more responsive.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>