1. Ranking high in search results pages is only half the battle - you have to get people to click through too.
More than often than not, the user's first experience of your brand is to read Google's presentation of your page's title and snippet, a slim piece of text devoid of context or visual identity.
A well-crafted snippet (usually based on some combination of title, meta and intro text tags) can whet the user's appetite for your brand and your content. On the other hand, a snippet that reads like a contrived piece of clever keyword stuffing could switch them right off.
Even clients of ours whose business model stands or falls on successful search performance are adamant that the text Google's results page displays about them should be on brand and on tone.
2. The writer is the best tagger
From both search and usability viewpoints, it's good practice to give every page on your sitemap a specific purpose and focus. For every page there should be clear answers to the questions: What is this page about? What do we want people to do here?
The writer of a particular page should understand that purpose and focus better than anyone. The tagging that goes with that page is an attempt to crystallise these things for the user, within very restrictive parameters. Pulling that off is a copywriting skill too.
3. It'll save you time and resource
If you have your main web page copy written in one place and your tagging done retrospectively somewhere else, there's an obvious inefficiency.
Writing and tagging the pages at the same time is likely to produce the most accurate tags - written while the content is still fresh - and save on employing resource to go through all the content again at a later stage.
Some writers may complain that this isn't part of their job. But our feeling is that the market will soon tell them that it is, or go elsewhere. Read our tips on how to write descriptions in tags.