You cannot watch a host you do not know about. Subdomains accumulate: a staging environment from two years ago, a vendor's CNAME, a marketing microsite somebody spun up for one campaign. Nobody wrote them down, and they are still resolving.
A survey starts from your apex domain, discovers the subdomains, resolves them, and builds an asset map you can read in one place.
When you inherited something
Acquisitions, team changes, and old projects all leave the same problem: a domain you are now responsible for and no inventory to go with it. A survey gives you the inventory in one run, so your first question becomes "which of these should still exist" instead of "what exists."
Feed it into everything else
The map is a starting point, not an endpoint. The hosts it resolves are the hosts worth scanning regularly, and the names it finds are worth watching for certificates. Running a survey first means your later work covers the whole surface rather than the handful of names you already remembered.
If there are subdomains that must always appear, name them as tracked subdomains and they get resolved every run, even when discovery misses them.
Where to go next
Domain Surveys covers the request and the map route. Scans is the usual next step for the hosts it turns up.