There’s many reasons to start a blog. But let’s start with the basics.
“Blog” is short for web-log, basically, and when they first came out they were essentially semi-automated managed content that made it easier for people to just write (and post photos) without worrying about the surrounding environment of the site (design, maintenance, etc.).
What were essentially web-diaries, however, soon became more intricately developed because, basically, all the platforms were open-source, and free.