Why is the site cached?

Question

PHP

Why cache the site to the level of whole html pages?
Well, let's imagine attendance of 100 people per minute, although I have little idea of ​​this for most sites.
So, even if one page, for hand-assed coders, it makes, for example, 20-30 queries to the database. Does the resource cost of working in real time outweigh the cost of maintaining the cache and tracking changes? Is this a relic of the past or is it really a working scheme? I repeat, this is not about highly visited sites, although there would be questions about those.

Answer 1

It is done for the sake of an instant response and for search engine optimization

Answer 2

1. To speed up the page load time. In WordPress, for example, I remember about 45 queries to the database from the box. After installing the plugin with caching, page loading speed is 2-3 times faster.
2. To save on servers. If the server is not very good, but there is no money for good capacities.
3. To optimize high-load applications - but object caching is more used there.