When the website schema is used, it can help Google resolve the ambiguity of which links on a site are the main sub-site links.
{
"@context": "https://schema.org/",
"@type": "WebSite",
"name": "example",
"url": "https://example.com/",
"hasPart" : [
{
"@type": "WebPage",
"name" : "About",
"url" : "https://example.com/about/"
},
{
"@type": "WebPage",
"name" : "Another Part",
"url" : "https://example.com/another/"
},
{
"@type": "WebPage",
"name" : "Google gives three site links today",
"url" : "https://example.com/last-but-not-least/"
},
{
"@type": "WebPage",
"name" : "Buy now, as determined keyword intent",
"url" : "https://example.com/by-intent/"
}
]
},
Depending on the site's structure, there may be no unambiguous indications of which pages should be considered the site links.
Note as of today the site links are shown as links directly under the description. Not the full panel: Google is in the process of cleaning up or simplifying the look of the SERP.