Writing proper English is very important for every person who wants to use the internet for earning money. There are many reasons why one needs to learn how to write Quality Content.
Few reasons:

The most important reason is that via writing content in proper English, one can start to earn lot of money on the internet via doing online copywriting jobs at freelance sites. Other than freelance sites, you can also use your copywriting skills to earn lot of money at other sites too.
One needs to know how to write and read English as on the internet, English is the primary way of communication. To make money online, you must need to have good command on your English writing skills.
For example: when you will communicate with other people at freelance sites, you will do it in English. All the processes like, registration, service provider/buyer communication etc are done using just one language which is English.
No matter what kind of work you choose to do at freelance sites, you must learn to write proper English. If you have 5 years of experience in C++ programming but you can’t write proper English, then you will face difficulties in getting work at freelance sites.
Buyers will think that you are not a professional programmer if you communicate with the buyer using badly written English.
So even if you are not doing copywriting jobs at freelance sites, you have to learn to write proper English in order to successfully earn money online.
Many buyers at freelance sites ignore the service providers who do not communicate in proper English. Buyers think of them as immature or unprofessional service providers. So the first and the most important thing to do is that you should work on your English Writing Skills.