It has nothing to do with brand and marketing - it's to do with franchising...
If I spend a lot of money / invest capital in a hotel (say an Indigo or an Intercon) I don't want another one opening up nearby / on my patch.
But, IHG want as a many hotels as they can in their system - therefore, the way for them to open another non-competing hotel nearby is to create a new brand for it. If they develop their own brand (as they did with voco) that costs IHG $$ - these niche brands are simply a low cost way to add new hotels to the system without p***ing off existing operators...