24 million people speak Dutch all over the world. It is an official language in the Netherlands and in Flanders, the neighboring northern provinces of Belgium. Furthermore, Suriname (South America), Aruba and the Dutch Antilles (Caribbean) consider it an official language. There are large Dutch-speaking communities in Guyana, Curaçao, Argentina, Brazil, Canada, Australia, South Africa, New Zealand, and the United States. This gives you quite a lot of opportunities for learning, practicing, and using the language.
Dutch is fairly easy to learn for speakers of English and German in particular as there are a lot of shared features.
Consequently, Dutch can also be a «gateway language» into learning West Germanic and other Germanic languages - which is great, especially if you are aspiring to learn many languages and become a polyglot.
Although one can enjoy a country’s culture without knowing the local language, knowledge of the language helps you truly immerse yourself into the culture.
Talking to locals, watching movies and reading books in Dutch, listening to Dutch music - you will enjoy it a lot more when you actually know the language.
Good knowledge of Dutch will allow you to work as a translator or interpreter, or a Dutch teacher. It is also very beneficial if you want to work and live in the Netherlands.
This reason is true for any language. Learning a foreign language boosts your memory and provides exercise for your brain to stay sharp and alert longer well into your senior years.
As you can see, there are many reasons to learn Dutch. It will allow you to travel to many countries in the world, meet new people, immerse yourself into the culture and maybe even get some new career opportunities.