From the docs:
The maintenance DB field is used to specify the initial database that
pgAdmin connects to, and that will be expected to have the pgAgent
schema and adminpack objects installed (both optional). On PostgreSQL
8.1 and above, the maintenance DB is normally called ‘postgres’, and
on earlier versions ‘template1’ is often used, though it is
preferrable to create a ‘postgres’ database for this purpose to avoid
cluttering the template database.
I'd add that once you are connected, it will give you list of other databases on that server you can also connect to. But until you connect to at least one database on the server, you can't fetch that list.
If it was just called "DB", that would suggest it is the only database on that server you can connect to, rather one of (potentially) several.