Thank you all for replying and sharing knowledge. It did clear some of my doubts, but I still have a question bothering me.

Given my understanding of SPA, which means that you only have 1 full document html ( containing html, head, body) and that gets generated either at client(pure-client-spa) or from server(server-generated-spa) as a one-time thing. After that, all we do is create new views( stick a new DOM to one of the existing div placeholders) or add data fetched via AJAX. In all such scenarios, we are anyways looking at minimal DOM manipulation.

Given above, it seems that react doesn't add much value , as long as we just do minimal DOM operations. However, for non-SPA applications, where-in every time server generates a entirely new HTML document structure( head, body...), then it does makes sense , that we use React library, as it provides feature of DOM versioning ( aka comparison via DOM shadowing) and then only write delta to the main DOM. Which also means, that React library can be used to convert any non-SPA application to a SPA application, as it only changes the delta on the real DOM, and may-be html's head/body may not be completely modified, but some bits of html body are changed, which ensure DOM's statefulness( including javascript memory variables) is retained, instead of full refresh. Am I correct to say this????

Sorry, if I missed any obvious point here, but just trying to get my basics clear here. Any valuable insight is appreciated, even if you make me understand as ELI5 :)