It is the metric closest to what a person means by the page has loaded. Google wants it under 2.5 seconds for 75% of visits. On a mid range phone on mobile data, most sites we measure miss it.
The causes are predictable: a slow first byte, a large unoptimised hero image, render blocking scripts and stylesheets, and fonts that hold up text. A build that sets image dimensions, serves WebP or AVIF, and preloads the one image that matters gets there.
On this site the LCP element on most pages is the headline, which is why it renders in under 1.5 seconds on a throttled phone.