When choosing the subject of the blog, you must choose a topic that will be primarily interesting for you. If you choose a topic that you like and touches you, then your blog will become popular, because you write interesting articles people that will benefit people. And this in turn will bring popularity and revenue to you.
The theme of the blog should be clearly spelled out.
When a visitor comes to your blog, he should immediately understand what your blog. I.e. in the header of your website should be specifically spelled out the title of the blog, description. This is very important because the reader needs to understand that he will find on your website. The theme of the blog needs to be specific. It is an important condition results in regular readers. The more precise the chosen theme of the blog – the more useful your site will be for visitors. And it's easy to explain – if you are restricted subject blog, then this theme will be disclosed to you in more detail and better than if you ran a blog on many topics. For example, think you have some disturbing you (for example, you need to resolve any important legal question). Who do you turn to a professional lawyer or person who knows a lot of things and a little versed in different subjects? Of course, you choose a specialist for solving your question. And in blogging — the more visitors will trust the information on the blog where there are specific themes, as the author of this blog works on a specific topic and is well versed in it.
The specific focus of the blog is welcomed by search engines.
There is one more important advantage — if you have topic-specific blog, then your website will be better in the search engines, which in turn also have a positive impact on website traffic and your income. Search engines pay attention to the theme of the blog, and if it is blurry, the promotion will be slow, the traffic to the blog will be low. In search engines all laid out on shelves – all sites are stored on certain topics. For example, look Yandex catalogue – the largest and trust catalogue in Runet. All of the sites that it contains, is strictly a certain section that matches the website theme. Your blog should also be in this directory if you are serious about blogging. And as you already know, to get there, the theme of your blog needs to be specific. In Yandex the catalogue includes only high quality websites will help to Orient the visitor to the selected topic. If your site gets into Yandex catalog, you can expect to increase attendance of your resource and the increase in ratings of the website.
I think you seriously begin the creation of your blog and want to unwind quickly. If Yes, then do not create a blog about everything, there had to be a certain theme of your blog. It is proven that only a narrow thematic blogs become really popular in the future bring good income to their owners. So take seriously the choice of the theme of the blog depends on it – whether you are interested to write articles and whether they are interesting to your readers! I think you will make the right choice!