When you do some stuff on your database, it can be useful disable, for a while, the foreign keys check. Here how to do it:
If you want to disable all constraints in the database just run this code:
-- disable all constraints EXEC sp_MSforeachtable "ALTER TABLE ? NOCHECK CONSTRAINT all"
To switch them back on, run: (the print is optional of course and it is just listing the tables)
-- enable all constraints exec sp_MSforeachtable @command1="print '?'", @command2="ALTER TABLE ? WITH CHECK CHECK CONSTRAINT all"
I find it useful when populating data from one database to another. It is much better approach than dropping constraints. As you mentioned it comes handy when dropping all the data in the database and repopulating it (say in test environment).