Why is building internal links to non-indexable pages not a good idea?

Internal links are primarily used to distribute PageRank and help search engines discover and understand the hierarchy of a website. If a page is non-indexable (e.g., due to a noindex tag or robots.txt exclusion), it cannot appear in search results. Therefore, directing internal links to such pages provides no SEO benefit, as these pages are intentionally hidden from search engines and cannot rank. While it doesn't necessarily harm your site, it's a wasted effort that could be better spent on indexable pages. Google can still crawl pages linked to non-indexable ones, so it doesn't inherently make crawling more difficult or cause de-indexing of the source page. Keyword ranking is also not directly affected by linking to non-indexable pages.