Automation is a feature designed to save you time and effort. It gives you an option to set the signature automatically for new or moved users limiting the space for human error.
This feature serves best to organizations with big turnover or frequent internal changes that result in signature changes and especially those that keep their user data up to date in G Suite Global Directory. Such organizations can either use Subscription (which will synchronize the signatures for the chosen users regularly for a flat monthly fee) or go for Credit Automation - which will synchronize the signatures only when there's a change in User Group affiliation (and for new users).
In other words, if you don't update individual users' signatures that frequently but have a higher user turnover, you might save a lot of time and effort with Credit automation.
Subscription Automation is more straightforward - it lets you add new or moved users to your previously purchased Subscription seats. It can also increase your Subscription seat count if necessary - you can manage the specifics, of course.
The Automation is a scheduled operation that happens daily but not right away after you do the changes. It also doesn't support new Send as addresses; you need to add them manually before they'll be processed.