Once a template is saved, there are three places you'll actually use it.
From a client profile
Open a client's profile and use the Actions menu to send an Email or SMS. Pick a saved template, and its merge tags resolve using that client's own data, their plan, and your agent info.
Bulk send from Reports
From Reports, use the Actions menu on a filtered list to send a template to everyone in it at once — useful for things like an AEP reminder to your whole book, or a birthday check-in to everyone turning 65 this month.
From an Automation
A Send Email or Send SMS step in an automation points at a saved template. Every time the automation runs, it sends that template — personalized for whichever client triggered it.
Wherever it's sent from, the template itself doesn't change — only the data filled into its merge tags does. See Merge Tags for how that resolution works.