Leaving religion out I guess I would say the benefit is a lifetime commitment of unconditional love, respect, & faithfulness. But one could argue that such a commitment can be made without marriage, or that marriage doesn't guarantee such a commitment... and they would be right.
Therefore, I think your personal beliefs about marriage will determine the "benefits" of such a union. My marriage is based on a particular set of beliefs which determine the "benefits" for me. Someone else might believe it's okay to marry 14 wives and his "benefits" are going to be completely different than mine

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other than that, it's fun, you get to use words like "wife" and "husband" when referring to your significant other, instead of "companion" or "partner" or "significant other".
i thought there was supposed to be a tax break for married couples filing jointly in the US, but when i figure it out, there is only like a $3 difference.