Why Study Cantonese?

Cantonese is one of the most widely spoken dialects in China and is almost as common as Mandarin among Chinese communities outside of China around the world.
Cantonese is the language of Hong Kong and the mother-tongue of business leaders all over South-East China and South-East Asia. As China's first markets open to the West, these are important areas for trade .
While Chinese is spoken by few foreigners (non-Chinese) as it is, there are proportionally even less non-Chinese Cantonese speakers than non-Chinese Mandarin speakers. There is great need and market for this language skill.
The Chinese population is rapidly expanding its online presence.
Cantonese grammar is surprisingly simple in many respects. There is no need to learn conjugations for verbs, and there is virtually no prefixes or suffixes to worry about learning. On the other hand, there are many root words which help in learning new vocabulary as they are used and reused in combination with other roots.

Some Challenges Westerners Learning Cantonese Face

Cantonese is a tonal language. Words must be spoken at the right pitch to convey the correct meaning or be understood. For speakers of languages that do not have tones, this adds a big level of difficulty.
The written language is not phonetic. It uses characters which have meaning, but do not suggest how to pronounce them. There are over 7000 characters in modern use, and to be functionally literate you need to memorize the meanings and pronunciations of about 2000 characters and their various combinations.
The spoken and written languages are different. There are dialectal characters, but they are not very widespread and are not entirely standardized. With most languages, learning how to read and write can be a help in learning the language. That is not true of Cantonese until a student is already fairly advanced in their oral skills.
Many Cantonese sounds are not present in Western languages.
There are very few cognates to words in Western languages. Speakers of Germanic languages (like English) and Latin based languages (French, Italian, Spanish, etc.) will not find many similar words in Cantonese at all.