The great thing about crypto is that there are no need for promises. Cryptocoins have
proof (Proof of Work). That's far better than any promise. Proof of work is what makes gold (or any commodity money in existense for that matter) valuable. Excluding fiat, all money from the beginning of time had proof of work in some form or another. The mere presence of gold is proof that someone, somewhere, put it real effort to obtain it. Proof of work is the most basic form of intrinsic value.
A certificate is a promise that such proof exists. As mentioned upthread, this was important for trade, as paper money (promise) is much easier to carry and store than heavy gold coins (proof). However, as I already stated, promises can, and will be, abused:
Gold certificates are actually a great example of deteriorating value to achieve volume. Goldsmiths learned very early that they could print many more certificates than they actually had gold to back up, because such few people ever bothered to redeem them. This kind of debasement was an early form of fractional reserve banking, and one of the first steps toward ultra-high volume trade.
Again though, such a promise is unnecessary in crypto, as the proof is right there in the coin.
I get that the proof of work is in the coin, but issuing promises(bank notes) based on gold reserves was still a reality at some point. The gold miners themselves don't really get a say on whether or not the person they sell to decides to issue promise notes based on their own holdings (in order to support things like a modern present-day economy) .. other than quitting mining. If that were to become our reality, then mining difficulty would just decrease to where anyone could sustain mining and continue issuing the same amount of "gold" to be produced otherwise. We've effectively blocked our own shot on this one (in a pretty neutral way).
As far as why anyone would want to issue promises for gold/monero reserves .. I can think of a few. The total supply of Monero is limited to 184.4 billion units. Currently we use numbers as high as 1000 trillion to define deficits of "promises". One number is bigger than the other, and we got there by issuing promises. Personally I don't see any specific reason (currently) to issue promises as well, because as you say the proof of work is right there in creating the monero .. but that same proof of work was also available by the very fact you have a 99.9% gold bar of "x" weight in the first place. All I'm saying is that I can see it happening again, because people will try to fit this into a world that they understand.