Well quite obvious that he will find larger audience when he register an international domain name. As most of the world’s population is non-English speaking and by getting an IDN registered, one can capitalizing on it superblyMost of the non-English speaking population use keyboards that support their native language. Thus, when they search in their local language, it makes sense for them when they search for a domain name, and they also prefer to go searching while using their own language.

Few Common benefits will be:

Larger Audience
Localization
Brand Protection
Availability

Further....

Establishing an email address, website or service with a fully IDN domain name shows commitment to non english speaking consumers, customers, partners, the media, and government regulators. It tangibly demonstrates desire to offer a completely service to them.
No English whatsoever stands between your brand and the consumer.
Fully-IDN email/web address eliminates the cognitive dissonance between your URL and your localized IDN brand name.
Email Address & URLs will be much, much more memorable.
Will enjoy enhanced brand protection for your localized IDN brand names.
Site`s Search Engine Optimization (SEO) is enhanced, because URLs will perfectly match with localized IDN brand names and messages.
Fully-IDN Email Address & Web URLs greatly help consumer protections against phishing attacks. IDN netizens (and your government) favor fully-IDN web addresses because IDN netizens are more easily able to spot phishing URLs when they`re in IDN.
Fully-IDN URLs & Email address accelerate the uptake of the IDN web for new netizens, with particularly good ease of use for rural people and cashed-up "silver surfers" (senior citizens).
Entering fully-IDN web addresses and email services on smartphones, tablets and Windows PCs (using ubiquitous IDN handwriting recognition) is much easier and faster even one can use voice command now.
With IDN he can touch base its customers in language of its choice.
Meaningful sentences can be composed in the very compact and concise language, creating rich URLs which marketers will discover new uses for.
Trademark and brand name Protection.

Dear UASG Team,

A CEO of one of the largest Company of India is asking me to give him three compelling reason on "Why Should he Adopt IDNs" while it create difficulties in marketing, communicating, handling and promoting different domains to his clients.

I have made probable answers to this, let me know if you wanna add some points to it.

A) Adopting IDNs will open the door to an extra US$10 Billion Economic Activity by capturing the large size of market who doesn`t know English in India (which he would not like to miss)
B) Deliver a better User Experience (UX), resulting in better customer satisfaction and retention
C) Increase in Indian Languages on Internet is the proof that future is Vernacular and you need to be UA ready in order to accept communication from your clients who are sending emails using IDNs.

Look forward for your views on this.

Regards,
Ashish