When your engine is running and the oil runs through the engine to lubricate it the oil eventually gets dirty and when that engine oil gets too dirty it looses it's lubrication properties and even picks up tiny metal shavings from your engine overtime which is normal.
If you never change your oil then you could eventually damage your engine and even throw a rod in the engine.
To have an engine last you a long time you need to give it proper care and checking and changing that oil is very essential to a long engine life.
Replacing an engine is not cheap but getting your oil changed is cheap compared to the cost of a damaged engine or a replacement engine and the labor cost to install that engine.
Always get your engine oil checked and change the oil at least every 3,000 miles and your engine will last a lot longer.