Why do gospel contradictions matter?

[Jenny A]

First off, I'm not a believer. But I constantly hear from fellow non believers that the gospels aren't reliable because they contain contradictions. Honestly, I don't understand why this argument is used. From my understanding, the gospels spread by word of mouth for at least 20+ years before being written down. And then mark was written down, which the others are based off of. So isn't the real reason they are unreliable due to them being circulated by conversation for over 20 years?

Please, correct me if I'm wrong. I'm happy to be educated.
(Any helpful links would be appreciated!)

Why are the contradictions used as arguments by atheists? Because many Christians claim the Gospels are 100% accurate either by divine inspiration or because the gospels were written by eye witnesses (a dubious if not down right fraudulent claim). If someone claims that something is absolutely accurate, there's nothing like a contradiction either internally (like what day was Jesus crucified on) or with what we know of the history of the ancient world (for example, the world wide census that never happened) to refute that claim.

Slightly less fundamentalists Christians will contend that the Gospels are "spiritually inerrant," a wishy-washy term I tend to think of as theologically inerrant. But there are theologically driven contradictions in the Gospels too.

Not necessarily a good reason, but I must admit that it's also rather fun to watch Christians tie themselves into logical knots trying to explain away the contradictions. It's an end in itself.

Finally, anyone genuinely interested in the likelihood of particular story in the Gospels, or any other historical document needs to weigh the evidence carefully. Contradictions between sources and contradictions between sources and the historical context are part of the evidence.