Why React?

Hello Everyone! I am new born baby in the world of web development as well to this site.

I am working for a company in which few days back they decided to work on some internal project and they decided to use ReactJS in it.

As I said that I am new born baby to this. I have several questions that's pops up in my mind. So, the first question that comes to my mind is Why React ? What were people using before when React was not there? What feature does React give us that others don't?

I have gone through ReactJS documentation where I saw example of SPA(Single Page Application). It convinced me a little bit that it's a good tool to build UI at client side very effectively. But then I read somewhere about SSR(Server Side Rendering). So, now question is what are the benefits of rendering at server side?

I understand that it give SEO benefits. But still question arises in mind that why ReactJS? What's React better at compared to other tools. Please help me understand it in a better way.

Thank you.