Every high school English teacher tells their students to write with their readers in mind, but so often we write with ourselves in mind--especially when we fancy ourselves to be writers. Writing for yourself is a fine approach for an aspiring novelist or blogger or poet while pursuing those aspirations. If there's no sure formula for success, you might as well write what you damn well please. If you enjoy what you write, you at least get the pleasure of creating it even if you don't get much money for creating it. Writing what you don't enjoy won't increase your slim chances of becoming a smash hit, and if you're have fun with your work that may come through to your eventual readers.
Most writing isn't part of novelist's or blogger's or poet's aspirations. Most writing advances the mundane stuff of life. Fancying yourself a writer is no excuse for making day-to-day writing into something inappropriately ambitious.
If you're emailing your company's IT support, they don't want to read your purple prose; IT doesn't want to read your poetry either, unless a bulleted list of relevant facts counts as poetry. If you're not selling freelance literary services, your customers don't want to be wowed by your mastery of dialogue. Your teacher wants the essay she assigned, not the one you feel called to write. Your boss doesn't want a moving depiction of the weather on the dawn of an eventful day when a quick email telling him he'll want to bring an umbrella would do. Your mom just wants to read about how cute her grandkids are--actually, just send her a picture.
There's nothing wrong with writing for yourself, at least not in the right time and place. You just need to know when and where.