They’re not important at all – spammers used to love filling Meta Keywords tags with anything and everything they dream of ranking highly for, whether deserved or not, so Google has pretty much ignored them for as long as Google has existed.
That’s why Google became so popular. Unlike the meta tag search engines prior, Google looked at backlink profiles and words visible on pages to determine authority and relevance, ignoring meta keywords tags because they were heavily spammed just for the benefit search engine impressions without accurately representing real on-site user experiences.
The presence of meta tags will of course increase page source code size, thus increases page loading time, which slightly impacts user experience but only for very few people such as those on very poor connections, so it has minimal negative effect on SEO – about the same as minifying (stripping out white space) vs not minifying the HTML source code of a page.
Meta Description tags do have a potentially positive impact, if done well, but this is generally not a huge factor – far less significant than page titles, although agencies do often treat meta descriptions as a priority, inflating their true value, sometimes due to lack of expertise, and sometimes due to deception.