When you Choose recipients for a campaign, selecting one or multiple tags allows you to filter and direct your content to the contacts that it will be most relevant to. *
By targeting your content instead of simply "sending to all", you will reach less contacts, but each contact will receive content that is more relevant to their individual needs and interests, which will help improve your contact trust and relationships. This should also reduce the likelihood of contacts unsubscribing due to feeling overwhelmed by content (spam), or if irrelevant content leaves them feeling like you don't know what they are looking for. 

* See a step-by-step guide to sending to contacts with particular tags.

In the case of a recurring campaign that sends on a scheduled date & time, contacts will start receiving content on the next scheduled send after having the tag added; and stop receiving it, if and when the tag is removed, ensuring that you are only sending content to contacts when it's relevant to them.*
For example: A Monthly Sold Report is sent on the 1st of each month to contacts with the tag of "Potential Seller", containing the same email (template), but containing updated, relevant listings and data for each month.

* If you would like to send content when it's immediately relevant to individual contacts, rather than on a set day and time for all chosen recipients, you can use a tag based trigger.

It's important to note when choosing tags that if you have multiple tags with similar names, like "Buyer" and "Buyers", you can select all of them within Choose recipients, however, as tag triggers and tag conditions only allow for one tag to be nominated, you may want to consider merging similarly named tags, to prevent confusion or excluding relevant contacts.