1) Why does Blogs are necessary?
For many reasons....
a) Google likes sites that have a lot of content, and blogs are perfect for this because you can create million of pages of content that you can index, which will bring you traffic. Sure, you can create millions of pages for ecommerce sites, as well, not just for blogs, but ecommerce sites usually have very little content on them and remember that google likes sites with a lot of content, so.... blogs are great for this
b) ranking in google is easier when the keyword you target is addressed in depth, and there's no better way of addressing keywords in depth than with blogs
c) google doesn't like dead websites (ie. websites that haven't been touched or updated for years), google likes to see that your site is "alive", so to speak, and there's no better way of showing google that your site is alive than by posting 1-2 articles a week to your website
d) people also don't like dead websites as they instill feelings of abandonment, of dead projects that no one cares about anymore
e) it turns new visitors into return visitors, which is a very important signal to google. If google sees that 9 out of 10 people that visit your site never come back they (google) will think that your site is not good, and your site might lose rankings as a result of "poor user experience". But by having a blog you publish content regularly and some of the people that have found your site interesting on their first visit will want to keep up with what you have to say, and the blog is a perfect opportunity for them to do so and when google sees that more and more people come back to your site... err... blog they will see that your returning visitors count increases and your site will gain better ranking as a subsequence.
f) having a blog allows you to rank for tougher keywords because content is important in SEO and if you can solve 20-30% of your ranking issues with content it means 20-30% less backlinks, therefore 20-30% less money that you'd have to spend on those links to rank a page without content for the same keywords that you target with your blog posts.
So, having a blog translates into less money that you have spend on backlinks, more keywords you can target for, better rankings that you can achieve by focusing on content, and more returning visitors. A blog is (still) useful no matter what business type you're having
2) I guess it is not a good idea to copy blogs from competitors \ other sites, Or actually it's possible with a little twist?
it's not ethical, but it works. It's up to you whether or not you'll do it, and suffer the (both good and bad) consequences, if any
3) If I am looking for a service that can write blogs in my niche, How does it called?
so, you are looking for a content writer?? There are 1000s of such "services", you can find them anywhere by just googling, or by simply checking out our marketplace:
https://www.blackhatworld.com/forums/content-copywriting.194/
I have been searching and only find PBN service\ guest posts and I am actually looking for unique blogs that will be in my site.
you are confusing me, man. PBNs and guest posts are backlinking opportunities, but you say "looking for unique blogs that will be in my site".
So, what exactly are you looking for? What are those "unique blogs" that will be in your site?? Content (articles, blog posts), or something else?