Just to add: I don't want to give the impression the SCHEMABINDING only produces benefits in UPDATE plans - it doesn't.
The key point is that the QO can apply many more optimizations to deterministic functions - that may be important in more complex SELECT statements, where the value can be evaluated once and a table-spool used to 'remember' the result of the function for later executions with the same parameters.
The caveat there is that the QO doesn't make any attempt to estimate the true cost of a scalar function - it uses a fixed cost. This may mean that it fails to apply worthwhile optimizations for relatively expensive functions.
Another advantage of SCHEMABINDING is the ability to persist and index computed columns:
ALTER TABLE dbo.TestTable ADD C AS dbo.ComputeNum(B) PERSISTED -- Fails
ALTER TABLE dbo.TestTable ADD C AS dbo.ComputeNumSB(B) PERSISTED -- Succeeds
The final 'win' is that even a non-persisted computed column based on a deterministic function can have statistics created on it:
ALTER TABLE dbo.TestTable ADD D AS dbo.ComputeNum(B) -- Not persisted, so succeeds
CREATE STATISTICS s on dbo.TestTable (C) -- Succeeds
CREATE STATISTICS s on dbo.TestTable (D) -- Fails