OEM arguably stands for Original Equipment by the Manufacturer. With OEM parts, you know they're going to fit and perform like the factory intended or requires. Aftermarket parts may sometimes offer advantages, but generally, over time, vehicles with original equipment will be worth more and have a better resale than one built up with aftermarket parts.
OEM It's not a replacement part, it's original equipment, and with that comes a certain measure of security and certainty that what you're getting is the standard used to measure anything else.