I've done my best to skim over the (huge) Applicant Guidebook and as far as I can tell, you don't need any reason per se to be granted a gTLD.
You have to be a corporation or organization with good legal standing and financial stability, and you have to slog through an epic and expensive beurocratic process. Your application can be rejected on various specific grounds, some technical and some political.
But I don't see anything that explains the purpose of gTLDs. So essentially, if there is no particular reason for you not to have one, you can have one.
It does at least say that you need to run a registry. So you can't just squat a gTLD. But I don't know, in general, how the prices and policies of the registrar are evaluated and enforced. It might be in there somewhere but it didn't jump out while scanning.
Registry fees alone will cost you a minimum of $100k a year even if you sell no domains. You also need to submit business plans and "proof" of economic viability; without those your application will be rejected. One factor that most people aren't aware of is that ICANN will require you to have enough money in reserve to keep the registry alive until a new buyer can be found in case things go bad; this can be as much as $4 per domain sold in cash as reserves. If you predict you'll sell 4 million domains you'd better account for the fact you'll need to keep $16M in the bank.
You have to be a corporation or organization with good legal standing and financial stability, and you have to slog through an epic and expensive beurocratic process. Your application can be rejected on various specific grounds, some technical and some political.
But I don't see anything that explains the purpose of gTLDs. So essentially, if there is no particular reason for you not to have one, you can have one.