Actually a common reason given for having corporate taxes is they're easier to collect since you collect from fewer payers.
The biggest problem is they obscure tax burden. All tax burden is ultimately born by individuals. If you levy on the corporation instead of the individual directly, then the burden just gets divided between clients, suppliers, owners, and employees, and how it get divides ends up depending upon the relative price elasticity of demand for each of their services. When taxes are levied on individuals directly, how the burden gets distributed can instead be codified in law.
At least in that sense, corporate taxes are somewhat anti-democratic. Politicians and voters are out for blood on this, but what they really want is for the owners to pay more tax. That can be better accomplished by just taxing capital gains and carried interest.
There is, of course, also tremendous deadweight loss in that business decisions are often dictated by what results in the best tax treatment rather than what is economically optimal. The fact that it actually makes financial sense to entirely relocate operations to other countries is itself a pretty big indictment of taxes like this. That's a huge amount of resources being allocated in a way that does not result in any improvement to the goods and services.
But, to the point of this treaty, if they're not going to eliminate corporate taxes and tax owners directly instead, the next best thing is at least make the taxes the same everywhere so the relative change in rates will stop dictating business decisions.