Add a batch of customers by hand
Customers → Bulk add gives you a grid. Enter customers row by row, then save them together.
Use it for a dozen or two new customers when you have no clean export to import.
Rows stand alone
Each row saves or fails on its own. A bad address on row 7 does not undo the rows that already saved. Fix row 7 and carry on.
When to import instead
For a full book of business coming out of another system, the import wizard is the better tool.
See also: “Import your customer list.”