Automotive Relays - What is the actual difference?
The environment in a car is quite different from that experienced by electronic components in a box on a table in your office.  There are several characteristics in particular that stress electronics components more in a car than a office environment:

Temperature.  A car still has to start on a cold winter morning in Nome or after baking in the hot July sun until mid afternoon in Yuma.  Not only do these extremes pose a challenge to operating ability, but the daily thermal cycling causes mechanical stresses.

Vibration.

Noisy power.  "12V" car power is nowhere near as nice as what a 12 V regulated power supply provides.  It can drop significantly during starting, goes to 14 V when the engine is running, and can have 10s of volts of nasty spikes on it.  For example, the normal 7805 regulator can be damaged by car power, even though it is rated to 35 V.  There is a special regulator series that is rated for car power, which of course costs more.

Humidity.  The relative humidity is unpredictable and can be at one extreme or the other for extended periods of time.



In addition to this, the end users generally don't tolerate failures as well in cars as they do in ordinary office appliances.

I can't say how much of this is relevant to your situation since you haven't told us what you are really trying to do.  However, keep in mind that there are millions of cars sold every year, so the volumes are high and there is considerable competition.  If manufacturers didn't have to pay for the extra robust characteristics of the components, they wouldn't.  The fact that a car company is willing to pay the extra $.50 for a relay that goes into a million cars, should tell you that it matters.  That's $500k they could save if they didn't.  Put another way, that's about 3-4 engineers for a year to come up with a cheaper alternative.