So you have a perfect platonic friendship but no sexual desire on either side? If you reached that point of marrying, what would be the advantage in being married?
I can think of plenty of advantages. Just for the record, advantages to me do not necessarily mean they are advantages, logical or desirable to others. ;~)
Pracaticality.
If you are close lifetime buddies you might get less static if you want to live in the same house.
But what I experienced with my close platonic friend was when he was in the hospital dying it would have been much better had we been married so I could have been there more and in on decisions. I still was but it was a major hassle and when the marriage solution dawned on me he was too ill and confused to really be competent. This hospital thing is also a reason many gays give for wanting to be able to marry.
If they have a pension entitlement to that :o)
Seriously though I can see a close friendship making a good marriage especially late in life when you want someone around each day.
Also some people may feel that marriage is needed even though it is still a platonic friendship to get the approval of family, friends and in particular local church if you are to live in the same house.