Over time, the moving parts on a bike wear down. Gear teeth slowly wear down by the forces and friction of the chain. The chain elongates as the inner parts wear and gaps widen.
Proper maintenance is essential for keeping your parts working optimally. Grit and insufficient lube will rapidly grind things down.
Parts wear together - as the chain stretches, the point of contact on the cog moves. Putting a new chain on a worn system can make a lot of noise, skip gears, and will wear the chain out faster.